adapt contributing documentation

[thawn]

Contributing should describe the process of contributing to this whitepaper:

• describe step by step (use screenshots) how to use the github web interface to
  • fork the repo
  • add or edit markdown files
  • submit a pull request
  • ask at least one other author for a review
• contributions should be submitted as pull requests where they can be discussed and then merged
• describe example of adding a uncertainty quantification method card in the docs/method_cards/ folder