Add sample commands for git-shell

Provide a 'list' command to view available bare repositories ending in
.git and a 'help command to display usage.  Also add documentation in
a README

Signed-off-by: Greg Brockman <gdb@mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/git-shell-commands/list

@@ -0,0 +1,10 @@
+#!/bin/sh
+
+print_if_bare_repo='
+	if "$(git --git-dir="$1" rev-parse --is-bare-repository)" = true
+	then
+		printf "%s\n" "${1#./}"
+	fi
+'
+
+find -type d -name "*.git" -exec sh -c "$print_if_bare_repo" -- \{} \; -prune 2>/dev/null