Docs for child processes

[psteckler]

Add a README for Child_processes.

Notes:

• there's a mention of the process kind for Libp2p_helper, which was added to master-qa, but not compatible, yet. This mention will become valid in the near future, then.
• there are some changes of coda to mina, but not for environment variables, which still in use in the codebase
• the exit 99 when the prover or verifier dies is changed to a failwith. I ran a demo node locally, killed the prover, and verified the daemon still exits. Reviewer: Coda_run.handle_crash gets called in this case, is that OK?

[nholland94]

I think the docs could use some more details here to be in line with our new documentation guidelines. In general, I'd prefer if we don't make code changes along with the documentation PRs; I'm not totally sure these code changes are benign.

[psteckler]

In general, I'd prefer if we don't make code changes along with the documentation PRs

I'll move those changes to a separate PR, if after reading my justifications, you're still not convinced.

[nholland94]

In general, I'd prefer if we don't make code changes along with the documentation PRs

I'll move those changes to a separate PR, if after reading my justifications, you're still not convinced.

Sorry for the delayed response. It's ok to leave them here, the justifications work for me.