Updated README with NMEA output

README.md

@@ -176,6 +176,12 @@ The output is published on the following:
     | `/fixposition/gnss1` | `sensor_msgs/NavSatFix` | as configured on web-interface | Latitude, Longitude and Height |
     | `/fixposition/gnss2` | `sensor_msgs/NavSatFix` | as configured on web-interface | Latitude, Longitude and Height |
 
+-   The Vision-RTK2 can also output an average GNSS-based LLH position (i.e., only GNSS, not Fusion) and heading estimate based on SOG and COG (i.e., the platform must be moving for it to be accurate) by utilizing several NMEA messages, which serves as an auxiliary output until Fusion is fully initialized. To do this, enable the NMEA-GP-GGA_GNSS, NMEA-GP-RMC_GNSS, and NMEA-GP-ZDA_GNSS messages. This message's output frequency equals the lowest frequency of any of these three message types. **Note: This output should only be used until Fusion is fully initialized.**
+
+    | Topic                | Message Type            | Frequency                      | Description                    |
+    | -------------------- | ----------------------- | ------------------------------ | ------------------------------ |
+    | `/fixposition/nmea` | `fixposition_driver/NMEA` | as configured on web-interface | Latitude, Longitude and Height |
+
 #### Vision-RTK2 IMU data
 
 -   From RAWIMU, at 200Hz