Add a table of contents to readme.md

Thought adding a streamlined TOC might make getting to some of the elements that are further down in the readme a little easier. I didn't include every header/section since it ended up being too long

Conform to rest of Readme style; move below intro

readme.md

@@ -6,6 +6,13 @@
 
 Even though JavaScript is single-threaded, IO in Node.js can happen in parallel due to its async nature. AVA takes advantage of this and runs your tests concurrently, which is especially beneficial for IO heavy tests. In addition, test files are run in parallel as separate processes, giving you even better performance and a isolated environment for each test file. [Switching](https://github.com/sindresorhus/pageres/commit/663be15acb3dd2eb0f71b1956ef28c2cd3fdeed0) from Mocha to AVA in Pageres brought the test time down from 31 sec to 11 sec. Having tests run concurrently forces you to write atomic tests, meaning tests that don't depend on global state or the state of other tests, which is a great thing!
 
+## Table of contents
+* [Usage](#usage)
+* [CLI Usage](#cli)
+* [Documentation](#documentation)
+* [API](#api)
+* [Assertions](#assertions)
+* [FAQ](#api)
 
 ## Why AVA?
 
@@ -550,4 +557,3 @@ Concurrency is not parallelism. It enables parallelism. It's about dealing with,
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