Add a built-in alias for 'stage' to the 'add' command

Add a built-in alias for 'stage' to the 'add' command

This comes from conversation at the GitTogether where we thought it would
be helpful to be able to teach people to 'stage' files because it tends
to cause confusion when told that they have to keep 'add'ing them.

This continues the movement to start referring to the index as a
staging area (eg: the --staged alias to 'git diff'). Also adds a
doc file for 'git stage' that basically points to the docs for
'git add'.

Signed-off-by: Scott Chacon <schacon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>