Create README/documentation for initial release

[phillipluther]

Once features bearing the initial-release tag are locked down and behaving as we want them, we'll need a helpful and thorough README to document usage of the package.

[marksmccann]

The API docs have been taken care of on #34 via automagic jsdocs. However, we probably want to include some boilerplate info/examples above the API. More specifically, I think it would be a good idea to enumerate the explicit features our component adds to node-sass and we should also include all the relevant badges.

Features

:heavy_check_mark: Compile multiple files via globs patterns :heavy_check_mark: Promise support :heavy_check_mark: Write compiled files (Node API) :heavy_check_mark: Write source maps (Node API) :heavy_check_mark: Output to a directory (Node API) :heavy_check_mark: Dynamically define output destination :heavy_check_mark: Drop-in, non-breaking replacement for node-sass :heavy_check_mark: Configuration file support (CLI) :heavy_check_mark: Combine multiple files into a single output

Badges

We will also probably want at least —

• version
• build
• dependencies
• test coverage

We can get those here once we release.

[marksmccann]

It also might be a good idea to call out usage examples so that we can easily answer this question by developers, "What is this, and why would I use it over node-sass"?

var results = await sass.render({
    file: 'src/**/*.scss',
    outFile: 'css',
    sourceMaps: true
});

> node-sass-extra src/**/*.scss --config ./node-sass.config.js
> node-sass-extra src/**/*.scss -o css/%b.min.css --output-style compressed

"Oh that's cool, that'll save me a ton of time!" "Sweet, I can do everything I want from the command-line"