Document the significance of the substring "testnet" in hostname

[dboreham]

Due to this code, what you choose for hostname can lead to (almost) endless confusion.
Suggest documenting this behavior in the main readme, or at least in the chains directory readme.
I'd be happy to supply a PR for said documentation.

[liangping]

Would be great, if you want to improve that

[dboreham]

Ok : #579