Add a readme

[jhaynes]

We need an overall readme for the project that we can iterate on. We also need component specific documentation within each rust project and as part of the build system.

[iliana]

Component-specfic docs likely belong in a top-level docs/ as Markdown.

[tjkirch]

I'd prefer to keep component-specific docs in the component's directory, because it seems more obvious to get a nice README when you navigate to the component rather than having to navigate back to the top level /docs. It's also easier to use tools like cargo-readme to keep rustdoc in sync with READMEs.

[zmrow]

Closing this in favor of #189 which adds user-level README's to the various places as well as the overall README