Document --appimage-portable-home

[probonopd]

Document --appimage-portable-home on https://github.com/AppImage/docs.appimage.org/blob/master/source/user-guide/portable-mode.rst.

Show an example using --appimage-portable-home.

Make clear that --appimage-portable-home and --appimage-portable-config are mutually exclusive (one should only use one of them, not both).

Maybe only document --appimage-portable-home because the other one seems to confuse people.

[TheAssassin]

What are these? Never seen/used these flags. Maybe provide a text, or send a PR?

[probonopd]

me@host:~$ /isodevice/Applications/appimagetool-x86_64.AppImage --appimage-help
AppImage options:
(...)
  --appimage-portable-home        Create a portable home folder to use as $HOME
  --appimage-portable-config      Create a portable config folder to use as
                                  $XDG_CONFIG_HOME
(...)

Portable home:

  If you would like the application contained inside this AppImage to store its
  data alongside this AppImage rather than in your home directory, then you can
  place a directory named

  /isodevice/Applications/appimagetool-x86_64.AppImage.home

  Or you can invoke this AppImage with the --appimage-portable-home option,
  which will create this directory for you. As long as the directory exists
  and is neither moved nor renamed, the application contained inside this
  AppImage to store its data in this directory rather than in your home
  directory

Note that only --appimage-portable-home is documented extensively, for a reason.