Contribute - Codebase Overview / Contribution Guide

After a great suggestion from a contributor, I've decided to open this issue to any contributors who would like to create a section completely devoted to contributing to the webpack source code, and an overview of the code.

• Codebase Overview - https://reactjs.org/docs/how-to-contribute.html
• Contribution Guide - https://reactjs.org/docs/codebase-overview.html

Both links show a great bulleted list of things that should be mentioned in this adaptation for webpack. I'm going to assign myself as advisor/mentor to this for contributors who would like to take a piece of one of these tasks. Breaking it down we'd need to:

• Section for contributing built into docs arch.
• Section for code overview built into docs arch.
• Start adding actual content for the pages/sections.