Improve "Getting started" documentation

[coopy]

http://formidable.com/open-source/victory/docs/ should have largely the same content as
https://github.com/FormidableLabs/victory#get-started:

• Add "Add Victory to your project: npm install victory --save" as a regular code block
• Add "Add your first Victory component" with the VictoryPie example as a Component Playground
• Add the "Including Components" section
• Add the "Components" section with links

Keep the "Contributing and Source" section as-is.