Add readme

*Work in progress. Will be added to as work progresses*.

Add readme that outlines installation, usage, and license, and links to expanded/supporting docs.

Status: **In progress**

Reviewers: @jeffkamo @kpeatt 
Ticket: When ready, fixes #27 
Linked PRs: whichever fixes #29 

## Changes
- Add readme
- Link to supporting docs

## How to test-drive this PR
- N/A
Yes, we should have an example on how to configure and how to extend. The key ideas: configure by setting a component’s variables *before* you import the component, and extend by adding/overriding styles or adding new modifiers *after* importing the component. The order is important, because each component will use its default values for variables unless the user sets them first. So:

```scss
// In your main stylesheet, we assume you have something like this. The order of imports is important: set Stencil configurable variables *before* importing Stencil components, and add to them *after* you import them.
@import 'variables'; // Do this first, so Stencil is aware of your settings.
@import 'stencil/dist/components/button/button'; // Import Stencil components before yours.
@import 'my-components/button'; // Add styles to Stencil components after importing them.

// In your variables partial, configure some variables for Stencil’s button:
$button-padding: 5px 10px;
$button-border-radius: 5px;

// In your own button component:
// Override or add to a Stencil style:
.c-button {
    border: 0; // override
    background-color: hsl(200, 70%, 50%); // add
}

// Extend with a new modifier, in this case for a button that uses the site’s brand color:
.c-button.c--brand-color {
    background: $brand;
    color: #fff;
}