Document implementing new instructions and apis

[spotandjake]

We should properly document how to implement new instructions and api's and work with the bindings.

Currently, my workflow has been:

• Find the instruction in src/binaryen-c.h
• Find the instruction in src/js/binaryen.js-post.js (If it doesn't exist in both, we need downstream changes to binaryen)
• Copy an existing instruction with similar requirements (parameter and return types)
• Implement the c bindings
• Implement the js bindings
• Implement the ml OCaml binding.
• Implement the mli OCaml interface
• Add smoke tests in test/test.ml

Good example reference (SIMD implementation): #222

Feel free to ask for help in the grain discord in the binaryen-ml channel.