Create PULL_REQUEST_TEMPLATE.md

.github/PULL_REQUEST_TEMPLATE.md

+## What's the purpose of this PR
+
+XXXXX
+
+## Which issue(s) this PR fixes:
+Fixes #
+
+## Brief changelog
+
+XXXXX
+
+Follow this checklist to help us incorporate your contribution quickly and easily:
+
+- [ ] Read the [Contributing Guide](https://github.com/ctripcorp/apollo/blob/master/CONTRIBUTING.md) before making this pull request.
+- [ ] Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
+- [ ] Write necessary unit tests to verify the code.
+- [ ] Run `mvn clean test` to make sure this pull request doesn't break anything.