Add section in README on CLI commands

`lotus server` and `lotus console` were added in
https://github.com/lotus/lotus/commit/f342c2095f79e2c3d354c87b0d3e427eb7c23546 and
https://github.com/lotus/lotus/commit/c5f38376c4e1bc822a29d96005f42bd6f970db25
respectively but are not documented in the README.

This adds a short description of the commands to the README as well as
documenting what the command assumes to be in place.