Clarify the installation instructions
Clarify the installation instructions

* Fixes incorrect file name referenced in "Setup"
* Clarifies how to install as a Gem _and_ actually require the gem in Jekyll

Hope this helps! :-)
aarongustafson/jekyll-crosspost-to-medium,daxlab/jekyll-crosspost-to-medium,aarongustafson/jekyll-crosspost_to_medium