Update README.md

Author: TomMeHo

README.md

@@ -109,8 +109,15 @@ Here are the screenshots of the supported media types in German English (I did n
 
 ![Properties of series](doc/properties_series.png) ![Properties of movies](doc/properties_movie.png)
 
+# How to contribute
+
+You're welcome to contribute, but please be aware of ...
+
+* You should provide a unit test for your changes with a good coverage. To do this, install ``nose2`` and ``coverage`` and execute the tests with 
+* I won't react immediately with checking your pull requests in.
+
 # References
 
 * Synology Video Station: 
     * [Help in Support Center](https://www.synology.com/en-en/knowledgebase/DSM/help/VideoStation/VideoStation_desc)
-    * [Product Page](https://www.synology.com/en-en/dsm/feature/video_station)
+    * [Product Page](https://www.synology.com/en-en/dsm/feature/video_station)