Add GitHub pull requests template (copied from Maven Enforcer plugin)

.github/pull_request_template.md

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+Following this checklist to help us incorporate your 
+contribution quickly and easily:
+
+ - [ ] Make sure there is a [JIRA issue](https://issues.apache.org/jira/browse/MJAVADOC) 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.
+ - [ ] Each commit in the pull request should have a meaningful subject line and body.
+ - [ ] Format the pull request title like `[MJAVADOC-XXX] - Fixes bug in ApproximateQuantiles`,
+       where you replace `MJAVADOC-XXX` with the appropriate JIRA issue. Best practice
+       is to use the JIRA issue title in the pull request title and in the first line of the 
+       commit message.
+ - [ ] Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
+ - [ ] Run `mvn clean verify` to make sure basic checks pass. A more thorough check will 
+       be performed on your pull request automatically.
+
+If your pull request is about ~20 lines of code you don't need to sign an
+[Individual Contributor License Agreement](https://www.apache.org/licenses/icla.pdf) if you are unsure
+please ask on the developers list.
+
+To make clear that you license your contribution under 
+the [Apache License Version 2.0, January 2004](http://www.apache.org/licenses/LICENSE-2.0)
+you have to acknowledge this by using the following check-box.
+
+ - [ ] I hereby declare this contribution to be licenced under the [Apache License Version 2.0, January 2004](http://www.apache.org/licenses/LICENSE-2.0)
+
+ - [ ] In any other case, please file an [Apache Individual Contributor License Agreement](https://www.apache.org/licenses/icla.pdf).
+