Add video streaming docs

[LorenzMeier]

@dogmaphobic Could you please move the documentation you wrote into the official dev guide? It's impossible to find right now 8)

https://github.com/mavlink/qgroundcontrol/tree/da022f97ccd7f32fdf7d0f3040acd667a59ca326/src/VideoStreaming

[dogmaphobic]

It's not just moving. I put that together over 2 and a half years ago as a stepping stone into video streaming :P It needs to be updated and made into an actual "documentation". I will handle it...

[LorenzMeier]

Thanks! The test instructions here don't seem to work for me on Mac OS:
https://github.com/mavlink/qgroundcontrol/tree/da022f97ccd7f32fdf7d0f3040acd667a59ca326/src/VideoStreaming

I'm currently updating this PR and fixing it along the way:
PX4/PX4-SITL_gazebo-classic#118

[LorenzMeier]

Ok, PX4/Firmware master has now working video streaming. So you can take it from there.