add a fanin pattern codes

gomore/04_fan_in/fan_in.md

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+# Fan-In Messaging Patterns
+
+===================================
+Fan-In is a messaging pattern used to create a funnel for work amongst workers (clients: source, server: destination).
+
+We can model fan-in using the Go channels.
+
+```go
+// Merge different channels in one channel
+func Merge(cs ...<-chan int) <-chan int {
+  var wg sync.WaitGroup
+
+  out := make(chan int)
+
+  // Start an send goroutine for each input channel in cs. send
+  // copies values from c to out until c is closed, then calls wg.Done.
+  send := func(c <-chan int) {
+    for n := range c {
+      out <- n
+    }
+    wg.Done()
+  }
+
+  wg.Add(len(cs))
+  for _, c := range cs {
+    go send(c)
+  }
+
+  // Start a goroutine to close out once all the send goroutines are
+  // done.  This must start after the wg.Add call.
+  go func() {
+    wg.Wait()
+    close(out)
+  }()
+  return out
+}
+```
+
+The `Merge` function converts a list of channels to a single channel by starting a goroutine for each inbound channel that copies the values to the sole outbound channel.
+
+Once all the output goroutines have been started, `Merge` a goroutine is started to close the main channel.