Document git-stash

This describes the git-stash command.

I borrowed a few paragraphs from Johannes's version, and added a few
examples.

Signed-off-by: Nanako Shiraishi <nanako3@bluebottle.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/cmd-list.perl

@@ -178,6 +178,7 @@ git-show-ref                            plumbinginterrogators
 git-sh-setup                            purehelpers
 git-ssh-fetch                           synchingrepositories
 git-ssh-upload                          synchingrepositories
+git-stash                               mainporcelain
 git-status                              mainporcelain
 git-stripspace                          purehelpers
 git-submodule                           mainporcelain