Add contribution guide to the repository

[simonsan]

I think it could be nice to add a small contribution guide to the repository stating what to take care about

• when making PRs
  • no force-pushing to keep commit history for easier review of changes
  • maybe even including a small PR template for this repo(?)
  • include new chapters into the SUMMARY.md for the book generation
  • allow edits of maintainers so we can actually work together with people on things
• when writing new articles
  • including and explaining the template
  • encourage people to make PRs to raise code/content quality of articles
  • write idiomatic Rust in their examples that builds (for playground examples in mdbook)

tbc