Create PULL_REQUEST_TEMPLATE.md

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:
+
+- [x] Make sure there is a [JIRA issue](https://issues.apache.org/jira/projects/ROCKETMQ/issues/) filed for the change (usually before you start working on it). Trivial changes like typos do not require a JIRA issue. Your pull request should address just this issue, without pulling in other changes - one PR resolves one issue. 
+- [ ] Format the pull request title like `[ROCKETMQ-XXX] Fix UnknownException when host config not exist`. Each commit in the pull request should have a meaningful subject line and body.
+- [ ] Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
+- [ ] Write necessary unit-test to verify your logic correction, more mock a little better when cross module dependency exist. If the new feature or significant change is committed, please remember to add integration-test in [test module](https://github.com/apache/incubator-rocketmq/tree/master/test).
+- [ ] Run `mvn -B clean apache-rat:check findbugs:findbugs checkstyle:checkstyle` to make sure basic checks pass. Run `mvn clean install -DskipITs` to make sure unit-test pass. Run `mvn clean test-compile failsafe:integration-test`  to make sure integration-test pass.
+- [ ] If this contribution is large, please file an [Apache Individual Contributor License Agreement](http://www.apache.org/licenses/#clas).