Update contributing.md (commons-app#1477)

Author: misaochan

CONTRIBUTING.md

@@ -14,6 +14,14 @@ changed. It is also good to prefix the first line with "area: " where the "area"
 is a filename or identifier for the general area of the code being modified.
 The body should provide a meaningful commit message.
 
+# Write Javadocs
+
+We require contributors to include Javadocs for all new methods and classes submitted via PRs (after 1 May 2018). This is aimed at making it easier for new contributors to dive into our codebase, especially those who are new to Android development. A few things to note:
+
+- This should not replace the need for code that is easily-readable in and of itself
+- Please make sure that your Javadocs are reasonably descriptive, not just a copy of the method name
+- Please do not use `@author` tags - we aim for collective code ownership, and if needed, GitHub allows us to see who wrote something without needing to add these tags
+
 # Write tests for your code (if possible)
 
 # Make sure the Wiki pages don't become stale by updating them (if needed)