Tizen target setup: Mention we support Tizen 3+.

Crosswalk has not supported Tizen 2.x for a long time; update the
statement here to make it clear.
Building Crosswalk: List the GBS URLs used for Tizen.

These URLs have been announced on the crosswalk-dev mailing list before,
but having them in a location that is more easily accessible also makes
sense.