add ros-get help

[alberth]

Many sub-command programs also have a sub-commmand 'help' to get a little guidance with the commands (usually a list of known commands). It's a bit more helpful than throwing a usage error what it currently answers:

usage: ros-get [-h] [-v]
               {install,update,list,remove,ws-create,ws-switch,ws-save,ws-list,ws-locate}
               ...
ros-get: error: invalid choice: 'help' (choose from 'install', 'update', 'list', 'remove', 'ws-create', 'ws-switch', 'ws-save', 'ws-list', 'ws-locate')

[Rayman]

We already have the following:

ros-get --help
ros-get install --help

What would ros-get help do? A more elaborate explanation?

[alberth]

Less elaborate, imho, for inspiration try

git help
svn help
hg help

It's not that strange that people will try ros-get help imho. Ideal is that it outputs something similar to what ros-get --help does (can't try that now, so not sure what it prints exactly, but a list of available commands goes a long way). At the very least, it could print "Sorry, no help topic available, try 'ros-get --help' instead" which is better than producing an error.

These programs also have help <topic> as command for more elaborate help. git jumps to the git-<topic> man-page at my system, svn and hg have elaborate topic pages as online documentation (and less man-pages.)
I think it's a bit early to support topics of help now, unless it's easy to reroute it to ros-get <topic> --help in some way. Maybe some relevant pieces of the manual could be used here eventually?