Provide documentation on the extension in the README.md

[clarkio]

See Browser Preview extension documentation as an example: https://github.com/auchenberg/vscode-browser-preview

Make sure to include the following:

• a description of the extension and it's intended use (also link to clips from the Twitch stream on 25 Jan, 2019 that describes this)
• a list of prerequisites/minimum requirements to run the extension
• a step by step guide to set up your local environment for other developers to clone, run and debug the extension themselves
• an animated gif showing the extension running through its features
• a list of features provided by the extension
• a list of configuration options a user can set as part of their VS Code settings for this extension

[clarkio]

Mostly completed in #40