Add a pull request GitHub template

[manics]

Proposed change

Add a pull request template .github/pull_request_template.md

Alternative options

Do nothing

Who would use this feature?

Reviewers would have more information when reviewing PRs. For example, one thing I think is particularly useful is for PRs to have explicit testing instructions where appropriate, especially for large changes which require non-trivial configuration settings to test.

(Optional): Suggest a solution

I've made a start here:
https://github.com/manicstreetpreacher/test-bots/blob/master/.github/pull_request_template.md
This is inspired by https://github.com/stevemao/github-issue-templates/blob/master/conversational/PULL_REQUEST_TEMPLATE.md

See also the discussion in #288

[choldgraf]

+1 from me, I think it'd be helpful

[sgibson91]

I think this is a great idea! Can I suggest checking out the Turing Way PR template too? https://raw.githubusercontent.com/alan-turing-institute/the-turing-way/master/.github/PULL_REQUEST_TEMPLATE.md

[manics]

Thanks all, I've finally opened a PR 😀 jupyterhub/.github#11
@trallard is helping too.