Update README with information on how to check out Nokee locally

[lacasseio]

Gradle doesn't provide a good enough experience for working across multiple repositories. For this reason, we use includeBuild and Git submodules. We have a good reason to choose this workflow which is not important for the read me. Users should execute git submodule update --init after a checkout.

However, if the user forgets to check out the submodules, Gradle seems to write temporary files in the submodules directories (as they are supposed to be includeBuild which then breaks the git submodule update --init command. We should include a note regarding reversing this without having to redo the clone.

[lacasseio]

See https://github.com/moonlight-stream/moonlight-ios#building as a good example.