Doc: Added GNU Free Documentation License link from licenses page.

-snippet already existed, but the page which quotes the snippet doesn't
 exist.

Task-number: QTBUG-34569
Change-Id: I9c1de16e09ba309a8076d6814742682adff6112c
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>