Add "Waiting For Goroutines To Finish" example.

Also link to it from the Channel Synchronization example

examples/channel-synchronization/channel-synchronization.go

@@ -1,6 +1,8 @@
 // We can use channels to synchronize execution
 // across goroutines. Here's an example of using a
 // blocking receive to wait for a goroutine to finish.
+// When waiting for multiple goroutines to finish,
+// you may prefer to [use a WaitGroup](waiting-for-goroutines-to-finish).
 
 package main