Readme: mention aspect ratios and resolution of files

[falue]

Something along the lines of:
The hdmi to rca and the rca to antenna convertors are fixed on the 16:9 image format.
However, sometimes old tvs squeeze that image to 4:3, other times they just cut off the image without distorting it.
So it is best to use a 4:3 image and see if it looks correct. If not, use the video fitting options on the webremote.
If that doesnt work, create an enlarged and distorted 16:9 video from your 4:3 video (no black bars, just fill the image). So the tv does the unsqueezing.

Output is about 720p. However, an old tv does show much less resolution (like 480p at max) so do not make the files unnecessarily large.