[Docs] Perf tools

Fixes #1814.
@petehunt @joshduck
The images look blurry (taken on retina then shrank down to 50%...)
Sounds good, but let's rewrite this introduction then:

```
React is usually quite fast out of the box. However, in situations where you need to squeeze every ounce of performance out of your app, React provides a [`shouldComponentUpdate()`](http://facebook.github.io/react/docs/component-specs.html#updating-shouldcomponentupdate) hook where you can add optimization hints to React's diff algorithm.

In addition to giving you an overview of your app's overall performance, ReactPerf is a profiling tool that tells you exactly where you need to put these hooks.
```