Write a guide for contributing new backends

[mjambon]

This should give a sense of what it takes for someone to add support for a new programming language in terms of:

• expertise needed
• time budget
• design and implementation guidance
• perhaps a list of experts who could be paid to do the job

[mjambon]

This was prompted by https://discuss.ocaml.org/t/atdgen-wizards-rust-backend/11642