[Work in progress] Improve ast doc + adding contributions "guidelines"

[theobat]

Things that I have to clarify (the list is also a WIP):

• Why is there a makefile ?
• Why is there no default stack.yml (required for some ide tools) ? And if the stance is to consider the stack.yml as local config why is it not in .gitignore ?
• What's the role of the *TypeDefinition in the AST
• How to start the graphql-wai part and document it

Actions left:

• more doc coverage in the AST file

[jml]

Thanks!

Please submit the contribution guidelines update as a separate PR. Also, we prefer rST for docs, but will accept Markdown if you don't want to do rST.

[theobat]

Right, I'll do that, closing for now.