Feature proposal: create a pull-request template

[HonkingGoose]

Which version of the book is affected?

Problem is present on master.
Problem is with the pull-request workflow on GitHub.

Describe the bug:

By opening a pull-request, you agree to license your work to the ProGit2 project under a Creative Commons license, and give permission to include the work in a print edition.
This is only mentioned in a CONTRIBUTING.md, that not all contributors might actually read before making a pull-request.

Steps to reproduce:

1. Go to the GitHub pull-request interface.
2. Open new pull-request.
3. Note that there is no notice given in the pull-request process that I'm licensing my work, or that I'm giving permission to include the work in a print edition.

Expected behavior:

1. Go to the GitHub pull-request interface.
2. Open a new pull-request.
3. The pull-request has a lightweight template, that explains that you grant a license to use your work under the Creative Commons license, and that you give permission to include the work in a print edition.
4. PR template contains a checkbox that the user is licensing their work to the project.
5. The pull-request template also warns against making a large rewrite (The CONTRIBUTING guide says that those changes should have a issue for discussion first).

Screenshots:

Additional context:

I noticed this issue when I was opening a PR for: #1477
And I thought, maybe we should just put this info straight into the PR process instead of requiring a contributor to go hunt around for the info.

Of course if this is something the project wants, I'll be more than happy to create a PR for this change. 😄 👍

[ben]

Yeah, this would be great. Adding a checkbox for licensing would help everybody know what they're agreeing to as well.

[HonkingGoose]

I'll go make a PR template then. 😄