add GStreamer + WHEP instructions (#4117)

README.md

@@ -894,6 +894,32 @@ If encryption is enabled, set `tls-validation-flags` to `0`:
 gst-launch-1.0 rtspsrc tls-validation-flags=0 location=rtsps://ip:8322/...
 ```
 
+GStreamer also supports reading streams with WebRTC/WHEP, although track codecs must be specified in advance through the `video-caps` and `audio-caps` parameters. Furthermore, if audio is not present, `audio-caps` must be set anyway and must point to a PCMU codec. For instance, the command for reading a video-only H264 stream is:
+
+```sh
+gst-launch-1.0 whepsrc whep-endpoint=http://127.0.0.1:8889/stream/whep use-link-headers=true \
+video-caps="application/x-rtp,media=video,encoding-name=H264,payload=127,clock-rate=90000" \
+audio-caps="application/x-rtp,media=audio,encoding-name=PCMU,payload=0,clock-rate=8000" \
+! rtph264depay ! decodebin ! autovideosink
+```
+
+While the command for reading an audio-only Opus stream is:
+
+```sh
+gst-launch-1.0 whepsrc whep-endpoint="http://127.0.0.1:8889/stream/whep" use-link-headers=true \
+audio-caps="application/x-rtp,media=audio,encoding-name=OPUS,payload=111,clock-rate=48000,encoding-params=(string)2" \
+! rtpopusdepay ! decodebin ! autoaudiosink
+```
+
+While the command for reading a H264 and Opus stream is:
+
+```sh
+gst-launch-1.0 whepsrc whep-endpoint=http://127.0.0.1:8889/stream/whep use-link-headers=true \
+video-caps="application/x-rtp,media=video,encoding-name=H264,payload=127,clock-rate=90000" \
+audio-caps="application/x-rtp,media=audio,encoding-name=OPUS,payload=111,clock-rate=48000,encoding-params=(string)2" \
+! decodebin ! autovideosink
+```
+
 #### VLC
 
 VLC can read a stream from the server in multiple ways (RTSP, RTMP, HLS, SRT). The recommended one consists in reading with [RTSP](#rtsp):