Add map data README.

modules/map/data/README.md

+# Map Data
+
+A bundle of correlated maps are organized as a directory, with a structure like:
+
+```txt
+<map_dir>                        # Defined by FLAGS_map_dir
+  |-- base_map.xml.txt           # Defined by FLAGS_base_map_filename
+  |-- routing_map.txt            # Defined by FLAGS_routing_map_filename
+  |-- sim_map.txt                # Defined by FLAGS_sim_map_filename
+  |-- default_end_way_point.txt  # Defined by FLAGS_end_way_point_filename
+```
+
+## Use a different map
+
+1. [Prefered] Change global flagfile: *modules/common/data/global_flagfile.txt*
+
+   Note that it's the basement of all modules' flagfile, which keeps the whole
+   system in one page.
+
+1. Pass as flag, which only affect individual process:
+
+   ```bash
+   <binary> --map_dir=/path/to/your/map
+   ```
+
+1. Override in the module's own flagfile, which generally located at
+   *modules/<module_name>/conf/<module_name>.conf*
+
+   Obviously it also only affect single module.
+
+   ```txt
+   --flagfile=modules/common/data/global_flagfile.txt
+
+   # Override values from the global flagfile.
+   --map_dir=/path/to/your/map
+   ```