Add Prettier config file

[PaulRBerg]

The goal is to get automatic formatting for Markdown files, which will be particularly helpful for tables. this is only a guess but I think that the current tables have been manually adjusted so that the length of the columns matches the content. If this is correct, Prettier will speed up the process.

Note: the Prettier config will just need to activate proseWrap.

Happy to open a PR if interested.