Add entry for DTLS 1.3

Since DTLS 1.3 was standardized in 2018 as rfc8446 and Firefox seems to support DTL1.3, it would be nice to get an overview of which browsers support DTLS 1.3 at this time. This is more or less related to WebRTC.
Chromes BoringSSL does not seem to implement DTLS 1.3 as of now and I am note sure about Safari or other browsers.