Add documentation for groups and up

ping @aanand 

I have a feeling you will need this eventually.  It's not done and as far as I can tell some of the described features aren't implemented yet, but it's better than nothing.  This is partially the result of me digging through the code figuring out how `docker groups` works, documenting it so that self-discovery will be easier for others down the line.

PTAL

Docker-DCO-1.1-Signed-off-by: Nathan LeClaire <nathan.leclaire@gmail.com> (github: nathanleclaire)