Add PR template

.github/PULL_REQUEST_TEMPLATE.md

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+<!--  Thanks for sending a pull request!  Here are some tips for you:
+
+In your PR, please ensure that you include the following information:
+* a description of the change
+* the motivation for the change
+* what issue it fixes, if any, in GitHub syntax (e.g. Fixes #XYZ)
+
+Both the description and motivation may reference other issues and PRs,
+but should be mostly understandable without following the links (e.g. when
+reading the git commit log).
+
+If you are updating the documentation, please do it in separate PRs from
+code changes. And use the following structure for your documentation PR:
+* The first commit in the PR should include the changes to the markdown files and the name of the commit should start with `docs:`.
+* The second commit should be the generated content from running `make gendocs`. The commit message should be `regenerate docs`.
+
+**PLEASE REMOVE THIS COMMENT BLOCK BEFORE SUBMITTING THE PR** (the bits
+between the arrows)
+
+-->