Add feed example

Former-commit-id: c2752225dd7c5aa19b8e1bd79f4513493e4cfbda [formerly cb091e596b79767eee5b312df94054d13be0f698]
Former-commit-id: 2471f1863d552209ee48324b2d26710b9ea246d8

sharding/p2p/feed_example_test.go

@@ -0,0 +1,42 @@
+package p2p
+
+import "fmt"
+
+// Feeds can tbe use to subscribe to any type of message.
+func ExampleServer_Feed() {
+	s, err := NewServer()
+	if err != nil {
+		panic(err)
+	}
+
+	// Let's wait for a puzzle from our peers then try to solve it.
+	type Puzzle struct {
+		Challenge string
+		Answer    string
+	}
+
+	feed, err := s.Feed(Puzzle{})
+	if err != nil {
+		// This shouldn't happen, but we should handle it anyway.
+		panic(err)
+	}
+
+	ch := make(chan Message, 5) // Small buffer size. I don't expect many puzzles.
+	sub := feed.Subscribe(ch)
+
+	// Always close these resources.
+	defer sub.Unsubscribe()
+	defer close(ch)
+
+	// Wait until we have a puzzle to solve.
+	msg := <-ch
+	puzzle := msg.Data.(Puzzle)
+
+	fmt.Printf("Received puzzle %s from peer %v\n", puzzle, msg.Peer)
+
+	if puzzle.Answer == "fourteen" {
+		fmt.Println("I solved the puzzle!")
+	} else {
+		fmt.Println("The answer isn't \"fourteen\"... giving up")
+	}
+}