GstCameraPlugin: add usage section to README

Signed-off-by: Rhys Mainwaring <[email protected]>

README.md

@@ -200,6 +200,38 @@ greater than one:
 MANUAL> param set SIM_SPEEDUP 10
 ```
 
+### 3. Streaming camera video
+
+Images from camera sensors may be streamed with GStreamer using
+the `GstCameraPlugin` sensor plugin. The example gimbal models include the
+plugin element:
+
+```xml
+<plugin name="GstCameraPlugin"
+    filename="GstCameraPlugin">
+  <udp_host>127.0.0.1</udp_host>
+  <udp_port>5600</udp_port>
+  <use_basic_pipeline>true</use_basic_pipeline>
+  <use_cuda>false</use_cuda>
+</plugin>
+```
+
+The `<image_topic>` and `<enable_topic>` parameters are deduced from the
+topic name for the camera sensor, but may be overriden if required.
+
+The `gimbal.sdf` world includes a 3 degrees of freedom gimbal with a
+zoomable camera. To start streaming:
+
+```bash
+gz topic -t /world/gimbal/model/mount/model/gimbal/link/pitch_link/sensor/camera/image/enable_streaming -m gz.msgs.Boolean -p "data: 1"
+```
+
+Display the streamed video:
+
+```bash
+gst-launch-1.0 -v udpsrc port=5600 caps='application/x-rtp, media=(string)video, clock-rate=(int)90000, encoding-name=(string)H264' ! rtph264depay ! avdec_h264 ! videoconvert ! autovideosink sync=false
+```
+
 ## Models
 
 In addition to the Iris and Zephyr models included here, a selection