Documentation improvement: better document client's config files

[gumb0]

The config files and genesis/<ForkName>.json files in https://github.com/ethereum/tests/tree/develop/Retesteth are not sufficiently documented, I think. There are some portions config files mentioned in https://github.com/ethereum/retesteth/wiki/Add-client-configuration-to-Retesteth but ideally I'd like to see what is the schema of these files, what must be included, what may be optionally included, what is not included, how retesteth uses these files exactly.

For example, currently it seems that these fields are required in config:

• name
• socketType
• socketAddress
• forks
• additonalForks (what is this, how is it different from forks?)

And in *.json:

• params
• accounts with a list of precompiles (this actually might be required only for aleth), but no other accounts

This kind of info would be useful in the guide "how to mare retesteth work with your client"

[winsvega]

https://github.com/ethereum/retesteth/wiki/Add-client-configuration-to-Retesteth#config-file

[winsvega]

https://ethereum-tests.readthedocs.io/en/latest/config-dir.html