[skip ci] add note about issue taking in contribution guide

CONTRIBUTING.md

@@ -10,6 +10,7 @@ Thanks for your interest in contributing to Tachiyomi!
 Pull requests are welcome!
 
 If you're interested in taking on [an open issue](https://github.com/tachiyomiorg/tachiyomi/issues), please comment on it so others are aware.
+You do not need to ask for permission nor an assignment.
 
 
 # Translations