Merge pull request #1020 from altheaden/pr-template

Add pull request template

.github/pull_request_template.md

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+<!--
+Thank you for your pull request.
+Please add a description of what is accomplished in the PR here at the top:
+-->
+
+<!--
+Below are a few things we ask you or your reviewers to kindly check. 
+***Remove checks that are not relevant by deleting the line(s) below.***
+-->
+Checklist
+* [ ] User's Guide has been updated
+* [ ] If this PR adds a new analysis task, it has also been added to the user's guide
+* [ ] Developer's Guide has been updated
+* [ ] API documentation in the Developer's Guide (`api.rst`) has any new or modified class, method and/or functions listed
+* [ ] Documentation has been [built locally](https://mpas-dev.github.io/MPAS-Analysis/latest/users_guide/quick_start.html#generating-documentation) and changes look as expected
+* [ ] `Testing` comment in the PR documents testing used to verify the changes
+
+<!--
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+