doc: command line interface (cli) dot-repository dwimmery

The Git cli will accept dot '.' (period) as the relative path
to the current repository. Explain this action.

Signed-off-by: Philip Oakley <philipoakley@iee.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitcli.txt

@@ -58,6 +58,10 @@ the paths in the index that match the pattern to be checked out to your
 working tree.  After running `git add hello.c; rm hello.c`, you will _not_
 see `hello.c` in your working tree with the former, but with the latter
 you will.
++
+Just as the filesystem '.' (period) refers to the current directory,
+using a '.' as a repository name in Git (a dot-repository) is a relative
+path for your current repository.
 
 Here are the rules regarding the "flags" that you should follow when you are
 scripting Git: