README: add demo ASCII cast

README.md

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+# Using Trezor as a hardware SSH agent
+
+
+[![Demo](https://asciinema.org/a/22959.png)](https://asciinema.org/a/22959)

README.rst

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-Using Trezor as a hardware SSH agent
-====================================
-
-Sample usage::
-
-	~/Code/trezor/trezor-agent $ ./trezor_agent -k home
-	ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBKJnIA4lKZ1hR2tNUOFmtc8MwAVR4oe0CP5QzSrviSi4joZSTzHcmazK0800w2aj132EEmf1kzl6Vf7h46iCeD8= home
-	~/Code/trezor/trezor-agent $ ./trezor_agent -k home > ~/.ssh/authorized_keys
-	~/Code/trezor/trezor-agent $ ./trezor_agent -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
-	$
-