Update README.md

Check [this comment](https://github.com/colinsf/cft-jekyll-site/commit/08fa2812a5809bf444a01be75fec9f257a68b300#commitcomment-7651500) for more details.  I don't know how to bundle `jekyll` config values into SASS variables, so this is required for local development since we have a couple of images define in SASS.