GUACAMOLE-3: Add contribution guidelines (CONTRIBUTING).
GUACAMOLE-3: Add contribution guidelines (CONTRIBUTING).

mike-jumper/incubator-guacamole-website,mike-jumper/incubator-guacamole-website,mike-jumper/incubator-guacamole-website,mike-jumper/incubator-guacamole-website,mike-jumper/incubator-guacamole-website