add readme immutability example

6-React/class-03-states/README.md

@@ -1,13 +1,19 @@
 # Class 03 States
 
-### Rendering Flow
+## Immutability
+
+An immutable value is one whose content cannot be changed without creating an entirely new value.
+
+## Rendering Flow
 
 - When the status changes
 - When the property changes
 - When parent component renders
 - Passing function is different from executing (infinite rendering)
 
-### The key must be unique and not random
+## Key Props
+
+The key must be unique and not random
 
 <pre>
 ["A", "B", "C", "D", "E"]
@@ -17,7 +23,7 @@
   0,   1,   2,   3,   4
 </pre>
 
-### Closures
+## Closures
 
 A closure is the combination of a function bundled together (enclosed) with references to its surrounding state (the lexical environment). In other words, a closure gives you access to an outer function's scope from an inner function. In JavaScript, closures are created every time a function is created, at function creation time.