Brief documentation for the mysterious git-am script

Brief documentation for the mysterious git-am script

The git-am script is nowhere called and nowhere (including itself)
explained, and the name isn't helpful either. For those like me who will
wonder what is it about, add some documentation stub for it to the
documentation.

I probably got something wrong and I don't feel like investigating all the
options - this is just kind of "emergency" docs.

Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <junkio@cox.net>