contributors guide

[gonsie]

• let's move the guidelines to a CONTRIBUTING.md file
• let's add some information about the repo layout and where new implementations should go
• let's add some information about the code and that it should only be a fragment of the algorithm, not a full-working example
• let's request that contributions be different in some way (so we don't get multiple examples for the same language/programming model/library).

[Heatherms27]

So far I'v split the markdown files into different folders containing markdowns of the different implementations. Thought this would make it easier to see what has and hasn't been done/what implementations are available, which may prove useful if someone would like to contribute.