Add pull request template

PULL_REQUEST_TEMPLATE.md

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+<!--
+Thanks for your contribution!
+
+Please take a quick look at those things down there. They're quite important.
+Really! We wrote them for you. Yes you! With utmost care. Read them.
+-->
+
+**For short term contributors:** we understand that getting your commits well
+defined like we require is a hard task and takes some learning. If you
+look to help without wanting to contribute long term there's no need
+for you to learn this. Just drop us a message and we'll take care of brushing
+up your stuff for merge!
+
+### Checklist
+
+- [ ] I read the [commit guidelines](http://coala.io/commit) and I've followed
+      them.
+- [ ] I ran coala over my code locally. (*All commits have to pass
+      individually.* It is not sufficient to have "fixup commits" on your PR,
+      our bot will still report the issues for the previous commit.) You will
+      likely receive a lot of bot comments and build failures if coala does not
+      pass on every single commit!
+
+After you submit your pull request, **DO NOT click the 'Update Branch' button.**
+When asked for a rebase, consult [coala.io/rebase](https://coala.io/rebase)
+instead.
+
+Please consider helping us by reviewing other peoples pull requests as well:
+
+- pick up any PR at <https://coala.io/review>
+- review it (check <https://coala.io/reviewing> for more info)
+- if you are sure that it needs work, use `cobot mark wip <URL>` to get it out
+  of the review queue.
+
+The more you review, the more your score will grow at coala.io and we will
+review your PRs faster!