docs: update README to reflect CI changes and improve structure

Author: nick-lai

README.md

@@ -1,21 +1,22 @@
 AssociativeArray
 ===
 
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-[![PHP 7 ready](https://php7ready.timesplinter.ch/nick-lai/associative-array/master/badge.svg)](https://travis-ci.org/nick-lai/associative-array)
+[![Scrutinizer Code Quality](https://scrutinizer-ci.com/g/nick-lai/associative-array/badges/quality-score.png?b=main)](https://scrutinizer-ci.com/g/nick-lai/associative-array/?branch=main)
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 **A lightweight associative array library for PHP.**
 
 ## Table of Contents
 
-- [Requirements](#requirements)
-- [Installation](#installation)
-- [Basic Usage](#basic-usage)
+- [AssociativeArray](#associativearray)
+  - [Table of Contents](#table-of-contents)
+  - [Requirements](#requirements)
+  - [Installation](#installation)
+  - [Basic Usage](#basic-usage)
     - [select()](#select)
     - [where()](#where)
     - [innerJoin()](#innerjoin)
@@ -32,7 +33,7 @@ AssociativeArray
     - [toArray()](#toarray)
     - [Array Access](#array-access)
     - [Traversable](#traversable)
-- [License](#license)
+  - [License](#license)
 
 ## Requirements