Add logo to readme. (#59)

* Add logo to readme.

Signed-off-by: Tom Wilkie <tom.wilkie@gmail.com>

* Fix image width

Signed-off-by: Tom Wilkie <tom.wilkie@gmail.com>

* Tweak readme.

Signed-off-by: Tom Wilkie <tom.wilkie@gmail.com>

* Add apache 2.0 license.

Signed-off-by: Tom Wilkie <tom.wilkie@gmail.com>