Add pull request template

[snowsignal]

What issue does this PR fix/close?

This closes one half of #20. (Issue templates still need to be added)

Describe what this PR does, briefly:

Make a cool template for future PRs! As a matter of fact, this PR uses the cool new template!

Check off the following items if you've done them:

• [X] This PR is ready for review (in other words, not a WIP)
• [X] New tests have been written to cover the new code in this PR (or your PR doesn't need tests)
• [x] Documentation has been added/changed to cover changes made (or your PR doesn't need new documentation)
• [ ] This PR has been tested on a robot (optional, but it helps)

  If your PR doesn't need new tests, explain why:

  This doesn't change any code.

  Do your changes break the public API of this library?

  They do not.