github: Add Pull Request template.

Provides pull request submitters with contributor documentation, and
prompts them to provide relevant information about testing, and how they
came to implement this change.

Sections are deliberately small so they don't crowd out the GitHub Pull
Request description text field.

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

.github/pull_request_template.md

@@ -0,0 +1,33 @@
+<!-- Thanks for submitting a Pull Request! We appreciate you spending the
+     time to improve MicroPython. Please provide enough information so that
+     others can review your Pull Request.
+
+     Before submitting, please read:
+     https://github.com/micropython/micropython/blob/master/CODEOFCONDUCT.md
+     https://github.com/micropython/micropython/wiki/ContributorGuidelines
+
+     Please check any CI failures that appear after your Pull Request is opened.
+-->
+
+### Summary
+
+<!-- Explain the reason for making this change. What problem does the pull request
+     solve, or what improvement does it add? Add links if relevant. -->
+
+
+### Testing
+
+<!-- Explain what testing you did, and on which boards/ports. If there are
+     boards or ports that you couldn't test, please mention this here as well.
+
+     If you leave this empty then your Pull Request may be closed. -->
+
+
+### Trade-offs and Alternatives
+
+<!-- If the Pull Request has some negative impact (i.e. increased code size)
+     then please explain why you think the trade-off improvement is worth it.
+     If you can think of alternative ways to do this, please explain that here too.
+
+     Delete this heading if not relevant (i.e. small fixes) -->
+