Add note about NEO-6M not working for some people.

README.md

@@ -35,7 +35,7 @@ the hardware setup.
 
 * [Adafruit Ultimate GPS Breakout]
 * Beitian BN-180
-* [u-blox NEO-6M]
+* [u-blox NEO-6M] (YMMV, see #9)
 
 If you have verified this application working with a module not listed here,
 please submit a PR adding it to the list.