Add detailed guide of how session keys work now

[laboon]

With the addition of rotate_keys RPC call, replacing the old way of passing in a mnemonic phrase from the command line, people have been asking about this new process.

Explain:

1. How rotate_keys works
2. Where it stores data
3. Why it is better

[lsaether]

Are you working on this @laboon or is it open for someone to pick up?

[laboon]

I can get it. Will submit something shortly.