port: net: add note about regions

README.md

@@ -68,6 +68,8 @@ If you want to use a PAL or JPN ROM instead, put them into the `data` directory
 * PAL: ROM name `pd.pal-final.z64`, EXE name `pd.pal.exe`.
 * JPN: ROM name `pd.jpn-final.z64`, EXE name `pd.jpn.exe`.
 
+Note that users with different ROMs can't play netgames with each other.
+
 Additional information can be found in the [wiki](https://github.com/fgsfdsfgs/perfect_dark/wiki).
 
 A GPU supporting OpenGL 3.0 or above is required to run the port.