add CONTRIBUTING.md

[Disservin]

No functional change

[vondele]

I think it is good to have this file, but I would prefer to keep it as concise as possible, with links to the wiki. The wiki is a better place to e.g. describe how to checkout a branch, it is also means we don't have duplicated information, and the wiki is somewhat easier to update.

There is also overlap with https://stockfishchess.org/get-involved/ so ideally these two places are also in sync.

[snicolet]

Looks good to me now.

Two suggestions, to make the style more similar to our Readme.md file:

• create a link section at the bottom of the file, and use link redirections there with Markdown syntax
• limit width of text to 80 characters, so that it can be read in a text editor

[Disservin]

Good suggestions, thanks.