Updated documentation for Resource message

Author: ayushgnv

msg/Resource.msg

@@ -1,5 +1,7 @@
 # This message is used to specify a resource, either by a URI or by its string content.
 
-string uri
-              
-string resource_string  # The content of the resource, only used if uri is empty
+string uri              # If uri field is empty, resource_string must not be empty.
+
+string resource_string  # An entity definition file passed as a string, only used if uri is empty.
+                        # If uri field is not empty, resource_string field will be ignored.
+                        # Simulators may support spawning from a file generated on the fly (e.g. XACRO).