Adds getting started instructions to README.
Adds getting started instructions to README.

I thought the README didn't accommodate new users particularly well. Added instructions to switch CSS from a default Jekyll site to use Jekyll Assets.

jeremy/jekyll-assets,mattbeedle/jekyll-assets,jekyll-assets/jekyll-assets,jekyll/jekyll-assets,jeremy/jekyll-assets,jekyll-assets/jekyll-assets,jekyll/jekyll-assets,jekyll-assets/jekyll-assets,mattbeedle/jekyll-assets,jeremy/jekyll-assets,mattbeedle/jekyll-assets,jekyll/jekyll-assets,mattbeedle/jekyll-assets,jeremy/jekyll-assets,jekyll/jekyll-assets