Contribution guideline

[htwangtw]

With a large number of PR, if on the contributor side they can do a few simple things that would be extremely helpful, such as

1. meaningful branch name - don't use main
2. ask people to keep their main for the sole purpose of tracking upstream, or just delete it
3. before making a new branch, make sure they are forking from the latest version of upstream

We can adapt some existing guidelines.