Add a repo description

[Mr0grog]

We’re trying to make sure every repo has a description (see ipfs-inactive/docs#55) and this repo is missing one. I don’t have permissions to add it, but I’d suggest something like:

Low-level types, functions, and values used by go-ipfs-cmds (and its predecessor, go-ipfs/commands)

As a side note, is there anything worth saying in the README to clarify why this isn’t just part of go-ipfs-cmds? Based on what it says now, it certainly sounds like it’s not meant to be used on its own.