MMAPS: Update mmaps_generator readme.txt with thread info

src/tools/mmaps_generator/Info/readme.txt

@@ -1,5 +1,8 @@
 Generator command line args
 
+--threads           [#]             Max number of threads used by the generator
+                                    Default: 3
+
 --offMeshInput      [file.*]        Path to file containing off mesh connections data.
                                     Format must be: (see offmesh_example.txt)
                                     "map_id tile_x,tile_y (start_x start_y start_z) (end_x end_y end_z) size  //optional comments"
@@ -63,4 +66,4 @@ movement_extractor 0
 builds all tiles of map 0
 
 movement_extractor 0 --tile 34,46
-builds only tile 34,46 of map 0 (this is the southern face of blackrock mountain)
+builds only tile 34,46 of map 0 (this is the southern face of blackrock mountain)