Add subscribe method to broadcast::Receiver

tokio/src/sync/broadcast.rs

@@ -758,6 +758,13 @@ impl<T> Receiver<T> {
         self.len() == 0
     }
 
+    /// Creates a new [`Receiver`] handle that will receive values sent **after**
+    /// this call to `subscribe`.
+    pub fn subscribe(&self) -> Self {
+        let shared = self.shared.clone();
+        new_receiver(shared)
+    }
+
     /// Locks the next value if there is one.
     fn recv_ref(
         &mut self,