Update README

README.md

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+This project started from https://github.com/TobiasBales/PlayuavOSD.git
+
+Supported platforms:
+-------------------
+  * Raspberry Pi 0-3 -- native mode (use VideoCore IV API)
+  * Linux with X11 -- use gstreamer overlay
+
+Supported autopilots:
+---------------------
+
+   * PX4 -- full support
+   * Ardupilot -- should work, but not tested
+   * any mavlink-based -- should work with small fixes
+
+Building:
+---------
+
+1. Build for Linux X11 (native build):
+  * `apt-get install libcairo2-dev gstreamer1.0-tools libgstreamer1.0-dev gstreamer1.0-plugins-base gstreamer1.0-plugins-bad gstreamer1.0-plugins-good gstreamer1.0-plugins-ugly`
+  * `make osd`
+
+2. Build for Raspberry PI (cross-build):
+  * `make build_rpi` (you need linux machine with docker installed)
+
+3. Build for Raspberry PI (native build):
+  * `make osd mode=rpi`
+
+Running:
+--------
+
+Default mavlink port which OSD listen is 14550.
+
+1. RPI: `./osd`
+2. Linux X11:
+   * Run test video stream (OSD will not work without video stream)
+   ```
+      gst-launch-1.0 videotestsrc pattern=black ! videoconvert ! \
+      video/x-raw,format=NV12,framerate=30/1,width=1280,height=720 ! \
+      x264enc bitrate=4000 tune=zerolatency ! rtph264pay config-interval=1 ! \
+      udpsink host=127.0.0.1 port=5600
+   ```
+   * Run OSD: `./osd`
+   * You should got screen like this:
+     ![gstreamer](scr1.png)
+
+
+Screenshots:
+------------
+![px4](scr2.png)