Pull request guideline

Here is the draft version of our pull request policy to be merged.

1. a test program to test this modification would be the best
2. Document this update in wiki if needed
3. check your patch by scripts/checkpatch.pl

• git diff master | ./scripts/checkpatch.pl -

3. name Pull request's title appropriately
4. rebase your patch with a single commit

• https://git-scm.com/book/en/v2/Git-Branching-Rebasing
• https://help.github.com/articles/about-git-rebase/