delete all mention of createChannels from the README/docs

[williamstein]

You deprecated createChannels completely, but it is the first thing mentioned in the docs in the README.

CoCalc directly uses enchannel-zmq-backend via createChannels, so this was a little bit confusing, and I've been reading all the source code to figure out how to rewrite our code not using createChannels. Doing so wasn't hard, and in fact my code is a lot better just using createMainChannel. That said, you might want to rewrite the docs to reflect this (good) change.

[captainsafia]

We can address this as part of upcoming doc writing work.