docs: New USAGE.md document

2 files changed, 175 insertions, 40 deletions

diff --git a/README.md b/README.md
index 07c5c3b..4f49dac 100644
--- a/README.md
+++ b/README.md
@@ -12,9 +12,10 @@
 
 [ **[Features](#features)** ] 
 [ **[Installation](#installation)** ] 
-[ **[Changelog](/docs/CHANGELOG.md)** ] 
+[ **[Usage](/docs/USAGE.md)** ] 
 [ **[Building](/docs/BUILDING.md)** ] 
-[ **[Hacking](/docs/HACKING.md)** ]
+[ **[Hacking](/docs/HACKING.md)** ] 
+[ **[Changelog](/docs/CHANGELOG.md)** ]
 
 <img src="https://tavianator.github.io/bfs/animation.svg" alt="Screenshot">
 <p></p>
@@ -126,7 +127,7 @@ $ bfs -nam needle
 <strong>bfs: error:</strong> Unknown argument; did you mean <strong>-name</strong>?
 </pre>
 
-`bfs` also includes a powerful static analysis to identify likely mistakes:
+`bfs` also includes a powerful static analysis to help catch mistakes:
 
 <pre>
 $ bfs -print -name 'needle'
@@ -142,48 +143,21 @@ $ bfs -print -name 'needle'
 <p></p>
 </summary>
 
-### `-exclude`
+For example, the `-exclude` operator skips over entire subtrees whenever an expression matches.
+`-exclude` is both more powerful and easier to use than the standard `-prune` action; compare
 
-The `-exclude` operator skips an entire subtree whenever an expression matches.
-For example, `-exclude -name .git` will exclude any files or directories named `.git` from the search results.
-`-exclude` is easier to use than the standard `-prune` action; compare
-
-    bfs -name config -exclude -name .git
+<pre>
+$ bfs -name config <strong>-exclude -name .git</strong>
+</pre>
 
 to the equivalent
 
-    find ! \( -name .git -prune \) -name config
-
-Unlike `-prune`, `-exclude` even works in combination with `-depth`/`-delete`.
-
----
-
-### `-hidden`/`-nohidden`
-
-`-hidden` matches "hidden" files (dotfiles).
-`bfs -hidden` is effectively shorthand for
-
-    find \( -name '.*' -not -name . -not -name .. \)
-
-`-nohidden` is equivalent to `-exclude -hidden`.
-
----
-
-### `-unique`
-
-This option ensures that `bfs` only visits each file once, even if it's reachable through multiple hard or symbolic links.
-It's particularly useful when following symbolic links (`-L`).
-
----
-
-### `-color`/`-nocolor`
-
-When printing to a terminal, `bfs` automatically colors paths like GNU `ls`, according to the `LS_COLORS` environment variable.
-The `-color` and `-nocolor` options override the automatic behavior, which may be handy when you want to preserve colors through a pipe:
-
-    bfs -color | less -R
+<pre>
+$ find <strong>! \( -name .git -prune \)</strong> -name config
+</pre>
 
-If the [`NO_COLOR`](https://no-color.org/) environment variable is set, colors will be disabled by default.
+As an additional shorthand, `-nohidden` skips over all hidden files and directories.
+See the [usage documentation](/docs/USAGE.md#extensions) for more about the extensions provided by `bfs`.
 </details>
 
 
diff --git a/docs/USAGE.md b/docs/USAGE.md
new file mode 100644
index 0000000..9f76f16
--- /dev/null
+++ b/docs/USAGE.md
@@ -0,0 +1,161 @@
+Using `bfs`
+===========
+
+`bfs` has the same command line syntax as `find`, and almost any `find` command that works with a major `find` implementation will also work with `bfs`.
+When invoked with no arguments, `bfs` will list everything under the current directory recursively, breadth-first:
+
+```console
+$ bfs
+.
+./LICENSE
+./Makefile
+./README.md
+./completions
+./docs
+./src
+./tests.sh
+./tests
+./completions/bfs.bash
+./completions/bfs.zsh
+./docs/BUILDING.md
+./docs/CHANGELOG.md
+./docs/HACKING.md
+./docs/USAGE.md
+./docs/bfs.1
+./src/bfs.h
+...
+```
+
+
+Paths
+-----
+
+Arguments that don't begin with `-` are treated as paths to search.
+If one or more paths are specified, they are used instead of the current directory:
+
+```console
+$ bfs /usr/bin /usr/lib
+/usr/bin
+/usr/lib
+/usr/bin/bfs
+...
+/usr/lib/libc.so
+...
+```
+
+
+Expressions
+-----------
+
+Arguments that start with `-` form an *expression* which `bfs` evaluates to filter the matched files, and to do things with the files that match.
+The most common expression is probably `-name`, which matches filenames against a glob pattern:
+
+```console
+$ bfs -name '*.md'
+./README.md
+./docs/BUILDING.md
+./docs/CHANGELOG.md
+./docs/HACKING.md
+./docs/USAGE.md
+```
+
+### Operators
+
+When you put multiple expressions next to each other, both of them must match:
+
+```console
+$ bfs -name '*.md' -name '*ING*'
+./docs/BUILDING.md
+./docs/HACKING.md
+```
+
+This works because the expressions are implicitly combined with *logical and*.
+You could be explicit by writing
+
+```console
+$ bfs -name '*.md' -and -name '*ING'`
+```
+
+There are other operators like `-or`:
+
+```console
+$ bfs -name '*.md' -or -name '*.sh'
+./README.md
+./tests.sh
+./tests/find-color.sh
+./tests/ls-color.sh
+./tests/remove-sibling.sh
+./tests/sort-args.sh
+./docs/CHANGELOG.md
+./docs/HACKING.md
+./docs/BUILDING.md
+./docs/USAGE.md
+```
+
+and `-not`:
+
+```console
+$ bfs -name '*.md' -and -not -name '*ING*'
+./README.md
+./docs/CHANGELOG.md
+./docs/USAGE.md
+```
+
+### Actions
+
+Every `bfs` expression returns either `true` or `false`.
+For expressions like `-name`, that's all they do.
+But some expressions, called *actions*, have other side effects.
+
+If no actions are included in the expression, `bfs` adds the `-print` action automatically, which is why the above examples actually print any output.
+The default `-print` is supressed if any actions are given explicitly.
+Available actions include printing with alternate formats (`-ls`, `-printf`, etc.), executing commands (`-exec`, `-execdir`, etc.), deleting files (`-delete`), and stopping the search (`-quit`, `-exit`).
+
+
+Extensions
+----------
+
+`bfs` implements a few extensions not found in other `find` implementations.
+
+### `-exclude`
+
+The `-exclude` operator skips an entire subtree whenever an expression matches.
+For example, `-exclude -name .git` will exclude any files or directories named `.git` from the search results.
+`-exclude` is easier to use than the standard `-prune` action; compare
+
+    bfs -name config -exclude -name .git
+
+to the equivalent
+
+    find ! \( -name .git -prune \) -name config
+
+Unlike `-prune`, `-exclude` even works in combination with `-depth`/`-delete`.
+
+---
+
+### `-hidden`/`-nohidden`
+
+`-hidden` matches "hidden" files (dotfiles).
+`bfs -hidden` is effectively shorthand for
+
+    find \( -name '.*' -not -name . -not -name .. \)
+
+`-nohidden` is equivalent to `-exclude -hidden`.
+
+---
+
+### `-unique`
+
+This option ensures that `bfs` only visits each file once, even if it's reachable through multiple hard or symbolic links.
+It's particularly useful when following symbolic links (`-L`).
+
+---
+
+### `-color`/`-nocolor`
+
+When printing to a terminal, `bfs` automatically colors paths like GNU `ls`, according to the `LS_COLORS` environment variable.
+The `-color` and `-nocolor` options override the automatic behavior, which may be handy when you want to preserve colors through a pipe:
+
+    bfs -color | less -R
+
+If the [`NO_COLOR`](https://no-color.org/) environment variable is set, colors will be disabled by default.