Add a simple pull request template.

.github/PULL_REQUEST_TEMPLATE.md

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+### Please consider the following when opening a pull request:
+
+- Link to the relevant JIRA ticket(s) and tag any relevant team(s).
+- Squash your changes down into one or more discrete commits.
+  In each commit, include description that could help a developer
+  several months from now.
+- If running `make upgrade`, run _as close to the time of merging as possible_
+  to avoid accidentally downgrading someone else's package.
+  Put the output of `make upgrade` in its own separate commit,
+  decoupled from other code changes.
+- Aim for comprehensive test coverage, but remember that
+  automated testing isn't a substitute for manual verification.
+- Carefully consider naming, code organization, dependencies when adding new code.
+  Code that is amenable to refactoring and improvement benefits all platform developers,
+  especially given the size and scope of edx-platform.
+  Consult existing Architectural Decision Records (ADRs),
+  including those concerning the app(s) you are changing and
+  [those concerning edx-platform as a whole](https://github.com/edx/edx-platform/tree/master/docs/decisions).