docs: add documentation

Author: Greg Bowler

Diff for: README.md

@@ -13,4 +13,60 @@ PHP Mess Detector (PHPMD) takes a given PHP source code base and looks for sever
 Usage
 -----
 
-// TODO
+```yaml
+name: CI
+
+on: [push]
+
+jobs:
+  build-and-test:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v3
+
+      - name: Composer install
+        uses: php-actions/composer@v6
+	  
+      - name: PHP Mess Detector
+        uses: php-actions/phpmd@v1
+        with:
+          php_version: 8.1
+          path: src/
+          output: text
+          ruleset: test/phpmd/ruleset.xml
+```
+
+Version numbers
+---------------
+
+This action is released with semantic version numbers, but also tagged so the latest major release's tag always points to the latest release within the matching major version.
+
+Please feel free to use `uses: php-actions/phpmd@v1` to always run the latest version of v1, or `uses: php-actions/[email protected]` to specify the exact release.
+
+Inputs
+------
+
+The following configuration options are available:
+
++ `version` - What version of PHPMD to use
++ `php_version` - What version of PHP to use
++ `vendored_phpmd_path` - Path to a vendored phpmd binary
++ `path` - A php source code filename or directory. Can be a comma-separated string
++ `ruleset` - A ruleset filename or a comma-separated string of rulesetfilenames
++ `output` - A report format
++ `minimumpriority` - rule priority threshold; rules with lower priority than this will not be used
++ `reportfile` - send report output to a file; default to STDOUT
++ `suffixes` - comma-separated string of valid source code filename extensions, e.g. php,phtml
++ `exclude` - comma-separated string of patterns that are used to ignore directories. Use asterisks to exclude by pattern. For example *src/foo/*.php or *src/foo/*
++ `strict` - also report those nodes with a @SuppressWarnings annotation
++ `args` - Extra arguments to pass to the phpmd binary
+
+If you require other configurations of PHPMD, please request them in the [Github issue tracker].
+
+*****
+
+If you found this repository helpful, please consider [sponsoring the developer][sponsor].
+
+[Github issue tracker]: https://github.com/php-actions/phpmd/issues
+[sponsor]: https://github.com/sponsors/g105b