[Cin] Creating AppImage

[Andrea paz]

@MatN
Thanks to Glitterball3
(https://www.cinelerra-gg.org/forum/postid/2022/), Phyllis added a
section in the manual on how to unzip an appimage so you can make
additions and changes to it. To complete the process, I found a way to
recompress the folders back into an appimage using the "appimagetool"
tool. However, this program is not in the repositories of all distros
(Fedora, for example), so it's not universal enough to put in the
manual. We can also do compression with "linuxdeploy", but I don't
know how to do it. Do you have any idea how to use linuxdeploy to
recompress the "squashfs-root" folder into a new appimage?