README.md should define a subcommand / explain what that is

[eyalroz]

Many (?) readers don't recognize the term "subcommand" - although they might actually be interested in the feature; they (read: I) simply haven't heard that term used before.

The README.md should include at least a one-liner definition of a subcommand, or perhaps illustrate with a command-line invoking a subcommand of a command. The first two sentences of the subcommands section of the gitbook could do.