Add yarn and bower installation to README

in response to issue dhg#370

README.md

@@ -6,10 +6,11 @@ Check out <http://getskeleton.com> for documentation and details.
 ## Getting started
 
 There are a couple ways to download Skeleton:
+- `yarn add skeleton-css`
+- `bower install skeleton` (deprecated)
 - [Download the zip](https://github.com/dhg/Skeleton/releases/download/2.0.4/Skeleton-2.0.4.zip)
 - Clone the repo: `git clone https://github.com/dhg/Skeleton.git` (Note: this is under active development, so if you're looking for stable and safe, use the zipped download)
 
-
 ### What's in the download?
 
 The download includes Skeleton's CSS, Normalize CSS as a reset, a sample favicon, and an index.html as a starting point.