Add default PR template and CONTRIBUTING

[reardonj]

Adds a new PR template to remind contributors about PR expectations, and a default minimal CONTRIBUTING doc to point users at the contributing guide.

For repositories that already have these files, they should continue to get used and these will only show up if a project doesn't have its own.

This ties in with the website contributing guideline updates: typelevel/typelevel.github.com#679

[reardonj]

ping @typelevel/maintainers if anyone has opinions on this.

[jducoeur]

Makes sense, but a question: does this display sensibly in the new-issue workflow? It's essentially invisible in the rendered view (since everything is a comment), so I'm simply unsure.

[reardonj]

Makes sense, but a question: does this display sensibly in the new-issue workflow? It's essentially invisible in the rendered view (since everything is a comment), so I'm simply unsure.

It's only new PRs, not new issues (issue can have multiple templates and didn't seem as nice to meddle with).

When you create a new PR, it will be in textbox showing the usual markdown, so it will be quite obvious to the PR creator. I put it in a comment as my experience is people chronically either don't delete these things, or treat them as a checklist to answer instead of reminders. Hopefully the comment keep it from showing up in the PRs regardless of what they do.

[reardonj]

Site is updated, if someone can merge this.