Consider adding a pull request template

[jeremiahpslewis]

Motivation: Yggdrasil is now a major piece of Julia infrastructure and a crossroads where lots of diverse parts of the Julia community interact. It also is steadily approaching 1k binaries, so it's achieved a nearly incredible critical mass.

In order to make life manageable for the maintainers and to ease the barrier to entry for new contributors (so they feel comfortable that they are making a contribution in the way the project wants one), we could add a pull request template with some of the following content:

• Recommendations for PR / commit naming (e.g. 'project prepend', [GR] Update Version)
• Reference to Julia code of conduct
• Quick links to common debugging issues?

[giordano]

I'm not sure a pull request template is the right tool for any of the points you want to raise. Something in the README might be more useful.