GEODE-2897 Add PR template for github

.github/PULL_REQUEST_TEMPLATE.md

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+Thank you for submitting a contribution to Apache Geode.
+
+In order to streamline the review of the contribution we ask you
+to ensure the following steps have been taken:
+
+### For all changes:
+- [ ] Is there a JIRA ticket associated with this PR? Is it referenced in the commit message?
+
+- [ ] Has your PR been rebased against the latest commit within the target branch (typically `develop`)?
+
+- [ ] Is your initial contribution a single, squashed commit?
+
+- [ ] Does `gradlew build` run cleanly?
+
+- [ ] Have you written or updated unit tests to verify your changes?
+
+- [ ] If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under [ASF 2.0](http://www.apache.org/legal/resolved.html#category-a)?
+
+### Note:
+Please ensure that once the PR is submitted, you check travis-ci for build issues and
+submit an update to your PR as soon as possible. If you need help, please send an
+email to [email protected].