ffmpeg is needed to record videos

[c-joly]

Hi @costashatz, here is in issue linked to the review openjournals/joss-reviews#6771

When playing with the examples, I figured out that ffmpeg is needed to save the videos. I did not find any mention to it in the doc.

Can you mention in the doc that ffmpeg is needed? And eventually reminds that it is installable easily with brew if it is not already installed.

[costashatz]

@c-joly feel free to re-open the issue if #216 did not fix this :) Thanks again!