Video pipline issues would be better with a box diagram

[hamishwillee]

http://dev.3dr.com/concept-video.html

1. This link is broken https://github.com/3drobotics/solodevguide/blob/master/book/video-overlay.html (gives github 404 error, so I believe this may have been deleted.

2. This pipeline would be better as a diagram

   GoPro -> HDMI cable -> HDMI encoder -> i.MX6 (h.264 encode/gstreamer) -> WiFi (UDP) -> Controller (h.264 decode/hdmi output/UDP relay to phone) -> WiFi (UDP) -> App

3. The viewing using VLC section (https://github.com/3drobotics/solodevguide/blob/master/book/concept-video.md#viewing-live-video) is confusing. It talks about using nc but doesn't explain what this does. It talks about your phone app, but it isn't clear what that has to do with running VLC on your computer.

I've tidied up the introduction and fixed up some other links, but more to be done!