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/help

@@ -0,0 +1,18 @@
+#!/bin/sh
+
+if tty -s
+then
+	echo "Run 'help' for help, or 'exit' to leave.  Available commands:"
+else
+	echo "Run 'help' for help.  Available commands:"
+fi
+
+cd "$(dirname "$0")"
+
+for cmd in *
+do
+	case "$cmd" in
+	help) ;;
+	*) [ -f "$cmd" ] && [ -x "$cmd" ] && echo "$cmd" ;;
+	esac
+done