Add the PR template file

PULL_REQUEST_TEMPLATE.md

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+<!-- 
+Thank you for submitting a pull request! Please add some info (if applicable) to give us some context on the PR.
+
+We will review the PR as soon as possible, leave feedback, add a tag, etc. and let you know what's going on.
+
+Cheers!
+
+The App Center team -->
+
+Things to consider before you submit the PR:
+
+* [x] Has `CHANGELOG.md` been updated?
+* [] Are tests passing locally?
+* [] Are the files formatted correctly?
+* [] Did you add unit tests?
+* [] Did you test your change with either the sample apps that are included in the repository or with a blank app that uses your change?
+
+## Description
+
+A few sentences describing the overall goals of the pull request.
+
+## Related PRs or issues
+
+List related PRs and other issues.
+
+## Misc
+
+Add what's missing, notes on what you tested, additional thoughts or questions.