Update contributing.md

Author: deltakosh

contributing.md

@@ -17,6 +17,7 @@ Every contribution has to come with:
 * A sample for the [Sample app](https://github.com/Microsoft/UWPCommunityToolkit/tree/master/Microsoft.Toolkit.Uwp.SampleApp) (If applicable)
 * Unit tests (If applicable)
 * You tested your code with SDK 10586 and SDK 14393
+* PR has to target dev branch
 
 ## General rules