Add contributing file

[jschepers]

• I added a CONTRIBUTING.md file with guidance on how to contribute.
• I adapted the README.md:
  • I deleted the "Adding documentation" and "How-to contribute" subsections because they are now in the CONTRIBUTING.md file.
  • I added a note to the "contributors" section

@behinger Do you think the content of the CONTRIBUTING.md file is sufficient? I was wondering whether:

• I should a note (either in the file or the section in the readme) to encourage people to contribute even if they are not super familiar with creating PRs, issues etc. But maybe this is also too much.
• I should add more information on formatting e.g. how to avoid being swamped with suggestions by reviewdog.

[behinger]

Looks very nice!

I think it reads very welcoming. If they hesitate to make a pr, they can always write an issue 👍

[jschepers]

Thank you for having a look. Then I will merge now.