Prevent bitrotting of code snippets in `README.md` files.

[jorisdral]

Originally suggested by @dnadales (#19 (comment)).

The README files contain a number of code snippets that could start bitrotting as we change the code in the packages. Possible solutions:

• Remove code snippets from the READMEs.
• Replace the READMEs by literate haskell files.
• Add a PR checklist item to update documentation.