Describe alternative to packaged extensions to add 'plugins' to application

[gbraad]

As described on the mailinglist by @alexlarsson :

---

So, each installation (e.g. /var/lib/flatpak, and
~/.local/share/flatpak as well as other customly added ones) can have
a subdirectory called "extension".

In this directory you can create
directories named $ref/$arch/$branch, and the contents of that will be
directly visible as the content of the plugins (i.e. what would be in
the "files" subdirectory of the deploy dir if you installed a packaged
version of the plugins).

So, for example, if you create a directory called:
~/.local/share/flatpak/extension/io.github.Hexchat.Plugin.Gerard/x86_64/18.08
Its contents would appear under /app/extensions/Gerard in the hexchat sandbox.

Note, the details above matches this from the hexchat manifest:

  "add-extensions": {
    "io.github.Hexchat.Plugin": {
      "version": "18.08",
      "subdirectories": true,
      "directory": "extensions",
      ...