Introducing a CONTRIBUTING guide

It's unlikely this point will help users. Users who would understand well-organized branches already know about the rebase command and/or would be careful in the first place. Users who do not understand this point would not be able to infer the intent by reading the provided link. It's probably safe to remove this point until you get enough pull requests to the project that you can provide a more detailed set of instructions for the specific types of issues that tend to arise.