Document new feature in androiddeplotqt

Explains how the new variable named ANDROID_EXTRA_PLUGINS works, by
interacting with the androiddeployqt configuration files.

Change-Id: Icd4f0d9e8abf441b59ea95d9e2de56e5857a9bfb
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>