doc: add explanation of sync.Mutex

README.md

@@ -36,3 +36,16 @@ select {
         // receive when c is blocked        
 }
 ```
+
+## sync.Mutex  
+
+Channels are great for communication, but what if we don't need communication?  
+
+What if we just want to make sure only one goroutine can access a variable at a time to avoid conflicts? This concept is called `mutual exclusion`, and the conventional name for the data structure that provides it is `mutex`  
+
+Go's standard library provides mutual exclusion with `sync.Mutex` and its two methods: `Lock` and `Unlock`  
+
+
+## sync.WaitGroup  
+
+https://tutorialedge.net/golang/go-waitgroup-tutorial/