Document that fastgen node requires separate steemd compilation

[youkaicountry]

Running a fastgen node requires that the steemd node has been compiled with BUILD_STEEM_TESTNET flag enabled, but it is not mentioned anywhere in the documentation.

[mvandeberg]

What is meant by "fastgen node"?

[theoreticalbts]

That's explained here in README.md

[mvandeberg]

Alright. The terminology I have been using is master/slave nodes. The master node is the one that does all of the special setup. Do we want to use fastgen, or stick with master/slave? This is probably a separate issue, but it is fairly minor.

[mvandeberg]

Specifically for this issue, all testnet nodes currently require BUILD_STEEM_TESTNET be enabled, not just fastgen nodes.

[theoreticalbts]

Do we want to use fastgen

I'm in favor of fastgen, since it summarizes what makes the node special (it generates blocks in the past faster than the normal block generation rate of 1 / 3s).