Android: Add documentation for including third-party libs

Since Qt 5.3.1, androiddeployqt will support deploying third-party
libraries in a relatively simple way. This needs to be documented,
though, since it will probably not be entirely intuitive.

Change-Id: I5ea5127b93f69d9d445cb6d29f330c8e8e895164
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>