README: specify GeoJSON as a dependency

3cac4a7 (views: respond with GeoJSON (#3), 2013-01-21) introduced
GeoJSON, but forgot to update the dependencies and installation
instructions.  Fix this.

Spotted-by: Ashok Gautam <ScriptDevil@zoho.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>