add pull request guideline

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,14 @@
+> Makes sure these boxes are checked before submitting your PR - thank you!
+>
+- [ ] Check that your javascript is following our [style guide](https://github.com/cocos-creator/fireball/blob/dev/.github/CONTRIBUTING.md) and end files with a newline
+- [ ] Document new code with comments in source code based on [API Docs](https://github.com/cocos-creator/fireball#api-docs)
+- [ ] If your pull request has gone "stale", you should **rebase** your work on top of the latest version of the upstream branch.
+- [ ] If your commit history is full of small, unimportant commits (such as "fix pep8" or "update tests"), **squash** your commits down to a few, or one, discreet changesets before submitting a pull request.
+
+Re: cocos-creator/fireball#
+
+Changes proposed in this pull request:
+- 
+- 
+
+@cocos-creator/engine-admins