[v2] default starter: update README

[shannonbux]

Summary

Lots of people read the default starter READme (it's one of the most used starters with Gatsby and has more stars than most other starters) and currently it only has installation instructions.

It'd be great to give people a "tour" of the starter structure and explain what each directory is for (directories that are unique to Gatsby).

Link to default starter source code: https://github.com/gatsbyjs/gatsby-starter-default

Motivation

People often ask about how Gatsby works so that they can have flexible knowledge to apply to new use cases and resolve bugs, and the default starter READme would be a great way to learn about how a Gatsby site works.