Update README w/TOC & link Contributing & MIT sections (#222)

Summary:
* Add a Table of Contents to README
* Link CONTRIBUTING section to CONTRIBUTING.md file
* Link License section to LICENSE file
* Change `Sonar` references to `Flipper` in CONTRIBUTING.md

![flipper-readme](https://user-images.githubusercontent.com/10098988/43982318-f201c870-9cba-11e8-90b7-ebc47854e641.gif)
Pull Request resolved: https://github.com/facebook/flipper/pull/222

Reviewed By: danielbuechele

Differential Revision: D9282868

Pulled By: passy

fbshipit-source-id: 2f1ff9b598b8a0afdde9039e5a2c4a157c48ed9c

CONTRIBUTING.md

@@ -1,4 +1,4 @@
-# Contributing to Sonar
+# Contributing to Flipper
 
 We want to make contributing to this project as easy and transparent as
 possible.
@@ -12,7 +12,7 @@ pushed to GitHub.
 For changes affecting both, native code and JavaScript, make sure to only create
 a single PR containing both parts of the code.
 
-Although the Sonar desktop app is only released for macOS right now, it is
+Although the Flipper desktop app is only released for macOS right now, it is
 possible to create Windows and Linux builds of the app. Please keep this in mind
 when dealing with platform-specific code.
 
@@ -51,5 +51,5 @@ run `yarn lint` and `yarn fix` to apply formatting fixes.
 
 ## License
 
-By contributing to Sonar, you agree that your contributions will be licensed
-under the LICENSE file in the root directory of this source tree.
+By contributing to Flipper, you agree that your contributions will be licensed
+under the [LICENSE](./LICENSE) file.

README.md

@@ -10,6 +10,25 @@ Flipper is a platform for debugging mobile apps on iOS and Android. Visualize, i
 
 ![Flipper](/website/static/img/splash@2x.png)
 
+## Table of Contents
+
+- [Mobile development](#mobile-development)
+- [Extending Flipper](#extending-flipper)
+- [Contributing to Flipper](#contributing-to-flipper)
+- [In this repo](#in-this-repo)
+- [Getting started](#getting-started)
+  - [Requirements](#requirements)
+- [Building from Source](#building-from-source)
+  - [Desktop](#desktop)
+    - [Running from source](#running-from-source)
+    - [Building standalone application](#building-standalone-application)
+  - [iOS SDK + Sample App](#ios-sdk--sample-app)
+  - [Android SDK + Sample app](#android-sdk--sample-app)
+  - [Documentation](#documentation)
+  - [Contributing](#contributing)
+  - [License](#license)
+
+
 ## Mobile development
 
 Flipper aims to be your number one companion for mobile app development on iOS and Android. Therefore, we provide a bunch of useful tools including a log viewer, interactive layout inspector, and network inspector.
@@ -86,7 +105,8 @@ Start up an android emulator and run the following in the project root:
 
 Find the full documentation for this project at [fbflipper.com](https://fbflipper.com/docs).
 
-## Contributing and license
+## Contributing
+See the [CONTRIBUTING](/CONTRIBUTING.md) file for how to help out.
 
-See the CONTRIBUTING file for how to help out.
-Flipper is MIT licensed, as found in the LICENSE file.
+## License
+Flipper is MIT licensed, as found in the [LICENSE](/LICENSE) file.