docs: Add contributing guidelines and pull request template

[MillironX]

Adds the documentation so that others know how this repo functions and fits within the nf-core GitHub Actions space.

PR checklist

• This comment contains a description of changes (with reason)
• [NA] If you've fixed a bug or added code that should be tested, add tests!
• Make sure your code lints (npm run lint)
• Make sure your code passes formatting checks (npm run format:check)
• [NA] Ensure the unit test suite passes (npm run test)
• [NA] Ensure the integration test suite passes (act -j example-usage)
• CHANGELOG.md is updated
• [NA] README.md is updated

[ewels]

Great stuff, thanks!

I almost wonder if some of these docs should go in the readme or somewhere more findable, but no strong opinion, this is totally fine as well.