Add testable example for GatheringCompletePromise

It is not immediately obvious how to do traditional (non-trickle)
ICE in Pion, and the current documentation does not make it
sufficiently clear.  Add a testable example for non-Trickle ICE.

gathering_complete_promise_example_test.go

@@ -0,0 +1,64 @@
+package webrtc
+
+import (
+	"fmt"
+	"strings"
+)
+
+// ExampleGatheringCompletePromise demonstrates how to implement
+// non-trickle ICE in Pion, an older form of ICE that does not require an
+// asynchronous side channel between peers: negotiation is just a single
+// offer-answer exchange.  It works by explicitly waiting for all local
+// ICE candidates to have been gathered before sending an offer to the peer.
+func ExampleGatheringCompletePromise() {
+	// create a peer connection
+	pc, err := NewPeerConnection(Configuration{})
+	if err != nil {
+		panic(err)
+	}
+	defer func() {
+		closeErr := pc.Close()
+		if closeErr != nil {
+			panic(closeErr)
+		}
+	}()
+
+	// add at least one transceiver to the peer connection, or nothing
+	// interesting will happen.  This could use pc.AddTrack instead.
+	_, err = pc.AddTransceiverFromKind(RTPCodecTypeVideo)
+	if err != nil {
+		panic(err)
+	}
+
+	// create a first offer that does not contain any local candidates
+	offer, err := pc.CreateOffer(nil)
+	if err != nil {
+		panic(err)
+	}
+
+	// gatherComplete is a channel that will be closed when
+	// the gathering of local candidates is complete.
+	gatherComplete := GatheringCompletePromise(pc)
+
+	// apply the offer
+	err = pc.SetLocalDescription(offer)
+	if err != nil {
+		panic(err)
+	}
+
+	// wait for gathering of local candidates to complete
+	<-gatherComplete
+
+	// compute the local offer again
+	offer2 := pc.LocalDescription()
+
+	// this second offer contains all candidates, and may be sent to
+	// the peer with no need for further communication.  In this
+	// example, we simply check that it contains at least one
+	// candidate.
+	hasCandidate := strings.Contains(offer2.SDP, "\na=candidate:")
+	if hasCandidate {
+		fmt.Println("Ok!")
+	}
+	// Output: Ok!
+}