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
Should we provide an example or recommendation on how to override/extend stencils?

For example...

```
// Include the stencil into your project...
@import 'stencil/button/button';
```

```
// Override the stencil configuration variables
$button-padding: 5px 10px;
$button-border-radius: 5px;

// Customize the button classes
.c-button.c--brand {
    background: $brand;
    color: white;
}
```