Set up a pull request template

[jasonaowen]

As we get closer to production, and have more functionality documented, we should be more mindful of the changes we make.

GitHub offers the ability to set up a pull request template which autofills the body of the new PR form. We can populate that with the things we want to be sure of, such as...

• is the functionality being changed documented? is that documentation updated in this PR?
• how have you tested this?
• does this affect the UI? do you have before & after pictures?

Also consider the templates in this repo as a source of ideas.

[brainwane]

Include: "have you used https://squizlabs.github.io/HTML_CodeSniffer/ or the Chrome accessibility toolbar to check for new a11y issues?"

[PaulMorris]

For a11y issues, include, if not prefer, using the axe-core browser extension, since our integration tests use the axe-core tool. Chrome browser developer tools use axe-core under the hood, so that may also be equivalent.