Nav Grid Server documentation and fixes

[DLu]

Adds README.md

Also fixes a few bugs:

• Moved check to see if a nav_msgs::OccupancyGrid map had been recieved to its proper place in the map_msgs::OccupancyGridUpdate callback.
• Add check to see if output_directory exists in order to avoid trying to call create_directories(".") which results in Invalid argument error.
• Actually use the value of frame_id when serving maps.