Advice for PR mechanics

[jennybc]

Most or all of this may already be present here, I'll check later. I'm just moving content from an issue in a repo I'm about to delete (https://github.com/tidyverse/contributing/issues/2).

Fork-and-clone >>> clone

Work in and make your PR from a non-master branch

Commit messages and PR titles should contain explicit, human-readable info, not indirect info that requires following a link.

• GOOD: "Add vignette on Important Topic X"
• BAD: "fixes #43"

Use the magic words to close or link to existing issues, but not in main commit message or PR title.

• In a commit, use the first line of the message for content, like "Add vignette on Important Topic X", then in the commit message body include "Fixes #43".
• In a PR, if the magic words aren't already used in one of the commits, include them in the PR description (not title).