Add Pull Request template; resolves #35. (#38)

Author: ruebot

.github/PULL_REQUEST_TEMPLATE.md

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+**The title of this pull-request should be a brief description of what the pull-request fixes/improves/changes. Ideally 50 characters or less.**
+
+* * *
+
+**GitHub issue(s)**:
+
+If you are responding to an issue, please mention their numbers below.
+
+* ex: #1
+* ex: #2
+
+# What does this Pull Request do?
+
+A brief description of what the intended result of the Pull Request (PR) will be, what problem it solves, technical details, and any possible side effects.
+
+Example:
+* Changes x feature to such that y
+* Added x
+* Removed y
+
+# How should this be tested?
+
+A description of what steps someone could take to:
+* Reproduce the problem you are fixing (if applicable)
+* Test that the PR does what is intended.
+* Please be as detailed as possible.
+* Good testing instructions help get your PR completed faster.
+
+# Additional Notes:
+
+Any additional information that you think would be helpful when reviewing this PR.
+
+Example:
+* Does this change require documentation to be updated?
+* Does this change add any new dependencies?
+* Could this change or impact execution of existing code?
+
+# Interested parties
+
+Tag (@ mention) interested parties.
+
+Thanks in advance for your help with the Archives Unleashed Project!