Add issue / PR templates

[ejpcmac]

Rationale

It is quite easy to forget to read the CONTRIBUTING.md or to miss the important parts. To ease contributions and avoid back-and-forth in PRs, a simple template could help with important information.

Concept

Let’s write a PR template that includes info like (non-exhaustive):

• reading the CONTRIBUTING.md,
• adding the proper copyright notices,
• making the PR editable so I can solve nits myself.

An issue template could also be useful to provide a frame.

Notes

See GitHub’s documentation on templates.