lcevcdecoder: Add README.md

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7330>

subprojects/gst-plugins-bad/ext/lcevcdecoder/README.md

@@ -0,0 +1,48 @@
+LCEVC Decoder Build Instructions
+--------------------------------
+
+1. Build and install the V-Nova's LCEVC Decoder SDK (LCEVCdec):
+
+- Checkout the repository: https://github.com/v-novaltd/LCEVCdec
+
+```
+$ git clone https://github.com/v-novaltd/LCEVCdec.git
+$ cd LCEVCdec
+```
+
+- Build and install the SDK
+- $BUILD_DIR and $INSTALL_DIR are local build and install directories
+
+```
+$ mkdir $BUILD_DIR
+$ cd $BUILD_DIR
+$ cmake  -G Ninja -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=$INSTALL_DIR ..
+$ cmake --build .
+$ cmake --install .
+```
+
+2. Build LCEVC decoder (lcevcdecoder) plugin for GStreamer:
+
+- You can now build the lcevcdecoder plugin by using the '-Dgst-plugins-bad:lcevcdecoder=enabled' meson flag
+- $BUILD_DIR and $INSTALL_DIR are local build and install directories
+- For example:
+
+```
+$ cd GStreaner
+$ meson setup $BUILD_DIR --pkg-config-path=$INSTALL_DIR/lib/pkgconfig -Dgst-plugins-bad:lcevcdecoder=enabled
+$ ninja -C $BUILD_DIR
+```
+
+3. Run GStreamer LCEVC decoder pipeline:
+
+- If the build was successful, you can test LCEVC decoding with the following pipeline:
+
+```
+$ gst-launch-1.0 filesrc location=/home/user/lcevc-sample.mp4 ! qtdemux ! h264parse ! openh264dec ! lcevcdec ! videoconvert ! autovideosink
+```
+
+- LCEVC decoding should also work with autoplugging elements:
+
+```
+$ gst-launch-1.0 playbin uri=file:///home/user/lcevc-sample.mp4
+```