Update README.md

Author: petrknap

README.md

@@ -2,15 +2,22 @@
 
 Enumerated type for PHP by [Petr Knap].
 
+* [What is Enum?](#what-is-enum)
+* [Why use Enums instead of Constants?](#why-use-enums-instead-of-constants)
+* [Usage of php-enum](#usage-of-php-enum)
+    * [Enum declaration](#enum-declaration)
+    * [Enum usage](#enum-usage)
+* [How to install](#how-to-install)
 
-## What is enum?
+
+## What is Enum?
 
 > In computer programming, an **enumerated type** (also called **enumeration** or **enum**, or **factor** in the R programming language, and a categorical variable in statistics) is a data type consisting of a set of named values called **elements**, **members**, **enumeral**, or **enumerators** of the type. The enumerator names are usually identifiers that behave as constants in the language. A variable that has been declared as having an enumerated type can be assigned any of the enumerators as a value. In other words, an *enumerated type has values that are different from each other*, and that can be compared and assigned, but which are not specified by the programmer as having any particular concrete representation in the computer's memory; compilers and interpreters can represent them arbitrarily.
 -- [Enumerated type - Wikipedia, The Free Encyclopedia]
 
-## Why use enum?
+## Why use Enums instead of Constants?
 
-Because it is safer than using constants.
+Because it is safer and less scary than using constants.
 
 ```php
 class MyBoolen