Readme: Format table for packagist.org

Author: tangrufus

README.md

@@ -94,6 +94,12 @@ $ php-matrix --mode=minor-only  ">=7.2 <8.4"
 
 ### `--source`
 
+Available sources:
+- `auto`: Use `offline` in `minor-only` mode. Otherwise, fetch from [php.net](https://www.php.net/releases/index.php)
+- `php.net`: Fetch releases information from [php.net](https://www.php.net/releases/index.php)
+- `offline`: Use [hardcoded releases information](resources/all-versions.json)
+[default: "auto"]
+
 <dl>
   <dt><code>auto</code> (default)</dt>
   <dd>Use <code>offline</code> in <code>minor-only</code> mode. Otherwise, fetch from <a href="https://www.php.net/releases/index.php">php.net</a></dd>