Readme: added note about mysql only support atm (#201)

Author: staabm

README.md

@@ -11,6 +11,9 @@ This extension provides the following features:
 * builtin support for `doctrine/dbal`, `mysqli`, and `PDO`
 * API to built the same features for your custom sql based database access layer
 
+**Note:**
+At the moment only mysql/mariadb databases are supported. Technically it's not a big problem to support other databases though.
+
 [see the unit-testsuite](https://github.com/staabm/phpstan-dba/tree/main/tests/data) to get a feeling about the current featureset.
 
 __Its really early days... and this libs has a few rough edges.__