added some infos on how the plugin works/how to use it

documents the insights from #26 into the README

Author: staabm

README.md

@@ -6,6 +6,14 @@
 
 * [PHPStan](https://phpstan.org/)
 
+This extension emits deprecation warnings on code, which uses properties/functions/methods/classes which are annotated as `@deprecated`.
+
+In case you don't own the code which you want to be considered deprecated, use [PHPStan Stub Files](https://phpstan.org/user-guide/stub-files) to declare deprecations for vendor files like
+```
+/** @deprecated */
+class ThirdPartyClass {}
+```
+
 
 ## Installation