Add/update CONTRIBUTING.md

[ccoVeille]

When adding/changing tools the only file that has to be manually updated is the one in tools/**/plugin.json.
After that the code generation package handles the rest (cargo run --package mdsf-codegen).

If developing locally you can use just precommit to handle codegen / formatting / linting.

I should probably document that in a CONTRIBUTING.md 😅

Originally posted by @hougesen in #635 (comment)