Add information on using plugins/imports on Android

Since Qt Creator doesn't know what dependencies the plugins
or imports used by your app has ahead of time, this needs to
be maintained manually. Add information about this to the
documentation.

Change-Id: I7285d93581b4e71243d2ad5537ff19c19ee52f83
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>