Improved readme

Author: lookyman

README.md

@@ -21,3 +21,9 @@ includes:
 services:
 	- Lookyman\PHPStan\Symfony\ServiceMap(/path/to/appDevDebugProjectContainer.xml)
 ```
+
+## Limitations
+
+It can only recognize pure strings passed into `ContainerInterface::get()` method. This follows from the nature of static code analysis.
+
+You have to provide a path to `appDevDebugProjectContainer.xml` or similar xml file describing your container. To generate it, you have to run the `Sensio\Bundle\DistributionBundle\Composer\ScriptHandler::installAssets` composer script, which is part of Symfony's postinstall scripts. And to be able to run it, you have to have a database ready. Which kinda defeats the purpose of static code analysis, but whatever..