Added MediaSDK backend usage page

MediaSDK-encode-decode-backend.md

@@ -0,0 +1,31 @@
+### Building OpenCV with MediaSDK support
+1. Install MediaSDK
+2. Make sure corresponding environment variable is set, Windows: INTELMEDIASDKROOT, Linux: MFX_HOME
+3. Build with `-DWITH_MFX` option turned on:
+    ```.cpp
+    cmake -DWITH_MFX=ON <path-to-opencv-sources>
+    cmake --build .
+    ```
+### Decoding
+Media containers are not supported yet, so it only possible to decode raw video stream stored to a file. It can be done with FFmpeg ([source1](https://stackoverflow.com/questions/19300350/extracting-h264-raw-video-stream-from-mp4-or-flv-with-ffmpeg-generate-an-invalid), [source2](https://superuser.com/questions/678897/extract-hevc-bitstream-with-ffmpeg)):
+```.sh
+# H264
+ffmpeg -i video.avi -vcodec copy -an -bsf:v h264_mp4toannexb video.264
+# H265
+ffmpeg -i in.mkv -c:v copy -bsf hevc_mp4toannexb out.h265
+```
+
+Then you can use _VideoCapture_ object to decode the resulting file:
+```.cpp
+VideoCapture cap(“video.264”, CAP_INTEL_MFX);
+```
+**Note!** 
+The file extension is important, because it will be used to [determine](https://github.com/opencv/opencv/blob/ec2409157857a5dabf159e6c052ec001e7110bf5/modules/videoio/src/cap_mfx_reader.cpp#L21-L31) the codec. It can be one of `.264`, `.h264`, `.mp2`, `.mpeg2`, `.265` or `.hevc`.
+
+### Encoding
+Use the _VideoWriter_ object:
+```.cpp
+int fourcc = VideoWriter::fourcc('H', '2', '6', '4');
+VideoWriter writer(filename, CAP_INTEL_MFX, fourcc, fps, frameSize, isColor);
+```
+Where `fourcc` can be one of `MPG2`, `H264`, `X264`, `AVC `, `H265` or `HEVC`.