Updates on branching model with the develop branch

[zhucaoxiang]

I am proposing to update our branching model inspired by this article.

There will be a new main branch named develop for testing functionalities before committing to the master. In the future, master will be used for hosting stable release and will only merge contents from the develop branch.

Both branches are protected. To push your changes into the develop branch. You have to do

1. Create a pull request;
2. Check the merge status, review your commits, make sure there are no conflicts;
3. Request someone else to review your sources;
4. Merge and close.

Right now I have set five administrators (besides myself), @lazersos, @landreman, @jcschmitt, @aaroncbader, @yasuhiro-suzuki. I guess this covers most of the institutions/countries. If you want to have administrative permission, please let me know.