copied GNU/Linux distro installation instructions from https://github.com/agraef/purr-data/wiki/Installation, and removed references between instructions for different platforms

This commit adds initial packaging to build and distribute Purr Data
using Flatpak.

This brings us in line with the [build
API](https://github.com/cgwalters/build-api) used by flatpak-builder.

This gets rid of some undefined behavior that occurs from sending an
int as an argument to dsp_add which reads it back as a t_int. On certain
archs this will produce garbage values in the perform routine which can
cause crashes.

Obs detected bugs

See merge request jwilkes/purr-data!313