[Cin] AppImage for CinelerraGG testing

[Phyllis Smith]

A little more testing has been done so I updated on the website, the
README.txt to notify users who might read this:
   https://cinelerra-gg.org/download/README.txt
and added a subdirectory "images" to complement the "tars" and "pkgs" for
versions.  I called it images in case flatpak shows up too.
Images contains the newer and older appimages and another README.appimage
based on the note from Andrea in the forum about usage and possibility of
having to install another library for Arch and others.

Interesting test results for a 1 minute 30 second video rendered as h265 hi
quality:
   binary build -- 50.458 seconds / 53.767 fps
  older image --  50.387 seconds / 53.843 fps
  newer image --  38.408 seconds / 56.044 fps
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