Document how to override elements in a junction project one depends on

The approach outlined in #321 (comment 65232874) should cover how to override how certain elements get built in a junction.

This is appropriate material for an example in the user facing example documentation.

Original report:

It should be possible to overlap elements already present in a junction

Use case:

we would like to use the freedesktop-sdk to build gnome-build-meta (see https://gitlab.gnome.org/GNOME/gnome-build-meta/issues/21), but is currently not possible because freedesktop-sdk already includes, for example, glib or gtk+

Do not build glib or gtk+ in gnome-build-meta is not an option as GNOME devs needs to always build the latest, while freedesktop-sdk project will stick with the current stable version

Seems this is not possible at the moment?