readme instructions

Building this:

Right now the only thing that needs to run is `sass`. The .html files expect a CSS
file to be in `css/design-board.css`. The `npm run sass` script does this. So, run:
  - `npm install` to make sure sass is installed
  - `npm run sass`

Then you'll be able to just open any of the .html files here in a browser.

**Note: If you installed your browser using a flatpak, it won't be able to see the
local CSS files because of how they restrict accesst to your local files. While this
is very cool, it can be kind of inconvenient for local HTML/CSS development.**