Contributions are welcome. Thank you for making the effort.
Found a bug and have a fix? Great. No need to create a different branch - just fork the repo, make the fixes, and create a Pull Request.
Usually it's best to first discuss a new feature via an issue or email.
To add a feature, fork the repo, create a seperate branch, and create a Pull Request. You do not have to add the feature to all three platforms (iOS, Android, Emscripten), but at least two platforms would be nice.