Add package functions to define and set the rendezvous method

Add exported functions to the snowflake client library to allow calling
programs to define and set their own custom broker rendezvous methods.

doc/using-the-snowflake-library.md

@@ -38,6 +38,60 @@ func main() {
 }
 ```
 
+#### Using your own rendezvous method
+
+You can define and use your own rendezvous method to communicate with a Snowflake broker by implementing the `RendezvousMethod` interface.
+
+```Golang
+
+package main
+
+import (
+    "log"
+
+    sf "git.torproject.org/pluggable-transports/snowflake.git/client/lib"
+)
+
+type StubMethod struct {
+}
+
+func (m *StubMethod) Exchange(pollReq []byte) ([]byte, error) {
+    var brokerResponse []byte
+    var err error
+
+    //Implement the logic you need to communicate with the Snowflake broker here
+
+    return brokerResponse, err
+}
+
+func main() {
+    config := sf.ClientConfig{
+        ICEAddresses:       []string{
+            "stun:stun.voip.blackberry.com:3478",
+            "stun:stun.stunprotocol.org:3478"},
+    }
+    transport, err := sf.NewSnowflakeClient(config)
+    if err != nil {
+        log.Fatal("Failed to start snowflake transport: ", err)
+    }
+
+    // custom rendezvous methods can be set with `SetRendezvousMethod`
+    rendezvous := &StubMethod{}
+    transport.SetRendezvousMethod(rendezvous)
+
+    // transport implements the ClientFactory interface and returns a net.Conn
+    conn, err := transport.Dial()
+    if err != nil {
+        log.Printf("dial error: %s", err)
+        return
+    }
+    defer conn.Close()
+
+    // ...
+
+}
+```
+
 ### Server library
 
 The Snowflake server library contains functions for running a Snowflake server.