Add PR template

.github/PULL_REQUEST_TEMPLATE.md

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+### Purpose
+
+Describe the purpose of this change. If there is an existing issue that is resolved by this pull request, please reference it in the form `Fixes #1234` where 1234 is the relevant issue number.
+
+### Screenshots
+
+If this is a GUI change, try to include screenshots of the change. If not, please delete this section.
+
+### Documentation
+
+If this is an enhancement, add a link here to the corresponding pull request on https://github.com/sourcethemes/academic-www or describe the documentation changes necessary.