scripts/qemu-gdb: Add brief comment describing usage

Add a brief comment describing how to use the debug support
from GDB.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1439574392-4403-5-git-send-email-peter.maydell@linaro.org

scripts/qemu-gdb.py

@@ -13,6 +13,9 @@
 # Contributions after 2012-01-13 are licensed under the terms of the
 # GNU GPL, version 2 or (at your option) any later version.
 
+# Usage:
+# At the (gdb) prompt, type "source scripts/qemu-gdb.py".
+# "help qemu" should then list the supported QEMU debug support commands.
 
 import gdb