[readme] add missing cv-bridge dependency

got the following error while trying to build the package, installing `ros-indigo-cv-bridge` fixed it.
```
  [rospack] Error: package 'lsd_slam_viewer' depends on non-existent package 'cv_bridge' and rosdep claims that it is not a system dependency. Check the ROS_PACKAGE_PATH or try calling 'rosdep update'
  Failed to invoke /opt/ros/indigo/bin/rospack deps-manifests lsd_slam_core
  
  [rospack] Error: package 'lsd_slam_core' depends on non-existent package 'cv_bridge' and rosdep claims that it is not a system dependency. Check the ROS_PACKAGE_PATH or try calling 'rosdep update'
    
```