adding lecture 28

lectures/28_concurrency/README.md

@@ -143,6 +143,7 @@ private:
 function (student thread, in this example). We pass the necessary shared data from the main thread to the
 secondary thread to facilitate communication.
 
+```cpp
 #define num_notes 10
 void student_thread(Chalkboard *chalkboard) {
 	Student student(chalkboard);
@@ -160,6 +161,8 @@ int main() {
 	}
 	student.join();
 }
+```
+
 - The join command pauses to wait for the secondary thread to finish computation before continuing with the
 program (or exiting in this example).
 - What can still go wrong? How can we fix it?