document version numbers

Author: Greg Bowler

README.md

@@ -26,6 +26,12 @@ jobs:
     # ... then your own project steps ...
 ```
 
+### 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/phpstan@v3` to always run the latest version of v6, or `uses: php-actions/phpstan@v3.0.0` to specify the exact release.
+
 Example
 -------
 
@@ -103,4 +109,4 @@ If you found this repository helpful, please consider [sponsoring the developer]
 
 [php-build]: https://github.com/php-actions/php-build
 [issues]: https://github.com/php-actions/phpstan/issues
-[sponsor]: https://github.com/sponsors/g105b
+[sponsor]: https://github.com/sponsors/g105b