Docs updates

Readme, contributing guide, docs edits, including:

• Updated links to point to ReadTheDocs pages instead of the repo Markdown pages
• Added package introduction to the readme
• New figures to readme and usage
• Fixed typos throughout
• Moved all the developer guidelines from readme.md to contributing.md
• Added basic install directions to readme

@rmcd can you especially look at the last two edit items? I thought it made sense to target the readme to a more general audience and move the developer-specific content to the Contributing Guide, what do you think?

AP