Short answer: You absolutely can (and should!) store things like tests, readme files, uplink scripts, Dockerfiles, etc in the App git repository. Keep doing what you’re doing!
Longer answer: Only the contents of the source code (.py and .yaml) files from the client_code and server_code directories in the repository are passed to the server worker processes, and even these are aggressively cached at every stage (based on the precise version of your app that is running). So there’s no need to avoid adding extra files to the repository 
.