myfirstcontrib: add 'psuh' to command-list.txt

myfirstcontrib: add 'psuh' to command-list.txt

Users can discover commands and their brief usage by running 'git help
git' or 'git help -a'; both of these pages list all available commands
based on the contents of 'command-list.txt'. That means adding a new
command there is an important part of the new command process, and
therefore belongs in the new command tutorial.

Teach new users how to add their command, and include a brief overview
of how to discover which attributes to place on the command in the list.

Since 'git psuh' prints some workspace info, doesn't modify anything,
and is targeted as a user-facing porcelain command, list it as a
'mainporcelain' and 'info' command.

As the usage string is required to generate this documentation, don't
add the command to the list until after the usage string is added to the
tutorial.

Reported-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>