Create PULL_REQUEST_TEMPLATE.md

Author: ShannonDing

PULL_REQUEST_TEMPLATE.md

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+## What is the purpose of the change
+
+XXXXX
+
+## Brief changelog
+
+XX
+
+## Verifying this change
+
+XXXX
+
+Follow this checklist to help us incorporate your contribution quickly and easily. Notice, `it would be helpful if you could finish the following 5 checklist(the last one is not necessary)before request the community to review your PR`.
+
+- [x] Make sure there is a [Github issue](https://github.com/apache/rocketmq/issues) filed for the change (usually before you start working on it). Trivial changes like typos do not require a Github issue. Your pull request should address just this issue, without pulling in other changes - one PR resolves one issue. 
+- [x] Format the pull request title like `[ISSUE #123] Fix UnknownException when host config not exist`. Each commit in the pull request should have a meaningful subject line and body.
+- [x] Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
+- [x] Write necessary unit-test(over 80% coverage) to verify your logic correction, more mock a little better when a cross-module dependency exists.
+- [ ] If this contribution is large, please file an [Apache Individual Contributor License Agreement](http://www.apache.org/licenses/#clas).