README: add sample usage

README.rst

@@ -1,2 +1,20 @@
 Using Trezor as a hardware SSH agent
 ====================================
+
+Sample usage::
+
+	~/Code/trezor/trezor-agent $ ./agent.py -k home
+	ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBKJnIA4lKZ1hR2tNUOFmtc8MwAVR4oe0CP5QzSrviSi4joZSTzHcmazK0800w2aj132EEmf1kzl6Vf7h46iCeD8= home
+	~/Code/trezor/trezor-agent $ ./agent.py -k home > ~/.ssh/authorized_keys
+	~/Code/trezor/trezor-agent $ ./agent.py -k home ssh localhost
+	Linux lmde 3.16.0-4-amd64 #1 SMP Debian 3.16.7-ckt9-3~deb8u1 (2015-04-24) x86_64
+
+	The programs included with the Debian GNU/Linux system are free software;
+	the exact distribution terms for each program are described in the
+	individual files in /usr/share/doc/*/copyright.
+
+	Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
+	permitted by applicable law.
+	Last login: Sat Jun  6 16:24:12 2015 from localhost
+	$
+