documentation: add 'see also' sections to git-rm and git-add

Pair up git-add and git-rm by adding a 'see also' section that
references the opposite command to each of their documentation files.

Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-add.txt

@@ -65,6 +65,9 @@ git-add git-*.sh::
 	(i.e. you are listing the files explicitly), it does not
 	add `subdir/git-foo.sh` to the index.
 
+See Also
+--------
+gitlink:git-rm[1]
 
 Author
 ------

Documentation/git-rm.txt

@@ -74,6 +74,9 @@ git-rm -f git-*.sh::
 	shell expand the asterisk (i.e. you are listing the files
 	explicitly), it does not remove `subdir/git-foo.sh`.
 
+See Also
+--------
+gitlink:git-add[1]
 
 Author
 ------