Add documentation for semantic_map_to_2d.

semantic_map_to_2d/README.md

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+This package contains a single map server node that publishes a downprojected metric map. 
+A service is provided for chaning which waypoint is currently being published.
+
+# Usage
+
+Start it with
+
+`rosrun semantic_map_to_2d semantic_map_2d_server`
+
+This then provides the service `/set_waypoint` with the following format [TODO: This should be moved to the nodes namespace...]
+
+Request:
+- `string waypoint` : The name of the waypoint to switch the map to.
+
+Response:
+- `bool is_ok` : If the map was switched or not
+- `string response` : Some textual description of what when wrong if not ok.
+
+The map server publishes the map as a `nav_msgs::OccupancyGrid` on the topic `/waypoint_map`.