[Cin] Issues related to Motion Plugin

[Andrea paz]

@Phyllis Smith
 > note to Andrea, I think we should make a footnote in the manual
mentioning the vid.stab ffmpeg filter used with ffmpeg as a starting
point for some motion stabilization beforehand.  > It would probably
save a lot of frustration on the part of users.  What do you think?

I put the notes on the ffmpeg plugin 'vid_stab'. I added more of
Georgy's tips (I didn't ask: is that okay with you Georgy?) and I also
attach 'normal.png' which corrects the 'Subtract' image that was wrong
(see other email).
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