doc: add explanation of the keywords used with goroutine

README.md

@@ -1,3 +1,38 @@
 # go-concurrency
 
-Aims to learn the knowledge about concurrency in Go
+Aims to learn the knowledge about concurrency in Go
+
+## Flow control with channels
+
+
+1. We can imagine that channel is a pipe, `chan<-` means to send a value into the channel. `<-chan` means to receive a value from the channel.  
+
+2. `select ... case` can execute sending value into a channel
+
+
+## Range and Close
+
+1. A sender can close a channel to indicate that no more values will be sent.  
+2. Receivers can test whether a channel has been closed by assigning a second parameter to the receive expression. `v, ok := <-ch`, `ok` is `false` if there are no more values to receive and the channel is closed.  
+3. The loop for `i:= range c` receives values from the channel repeatedly until it is closed.  
+4. Only the sender should close a channel, never the receiver. Sending on a closed channel will cause a panic.  
+5. Channels aren't like files; you don't usually need to close them. Closing is only necessary when the receiver must be told there are no more values coming, such as to terminate a `range` loop.  
+
+
+## Select 
+
+1. The `select` statement lets a goroutine wait on multiple communication operations  
+2. A `select` blocks until one of its cases can run, then it executes that case. It chooses one at random if multiple are ready  
+
+## Default selection
+
+1. The `default` case in a `select` is run if no other case is ready.  
+2. Use a `default` case to try a send or receive without blocking  
+```
+select {
+    case i:= <-c:
+        // use i
+    default: 
+        // receive when c is blocked        
+}
+```