Lack of Discovery-related configuration instructions

[1033309821]

Discv5 is disabled by default in Nethermind, but if I want to start the discv5 protocol, I can't find the relevant startup method.
Finally, we read the source code and found the corresponding startup method, for example: "DiscoveryVersion" : "ALL"
Where DiscoveryVersion is the corresponding configuration parameter.

[kamilchodola]

@flcl42 Can you add some docs for Discv5?

cc @rubo