Update installation instructions (#63)
Update installation instructions (#63)

* Update README.md with note to include gem from GitHub

The rubygems version isn't kept up to date and is currently broken in rails 5.0.
When included directly from github master branch, it works without problems.

* Create separate installation section in README

* Clarify wording about rubygems version

* Use github shorthand

Davidzhu001/webpacker,rails/webpacker,nickpith/webpacker,rmckayfleming/webpacker,usertesting/webpacker,rmckayfleming/webpacker,usertesting/webpacker,shakacode/webpacker_lite,rossta/webpacker,rossta/webpacker,gauravtiwari/webpacker,rossta/webpacker,rails/webpacker,nickpith/webpacker,gauravtiwari/webpacker,rossta/webpacker,Davidzhu001/webpacker,rmckayfleming/webpacker,Davidzhu001/webpacker,nickpith/webpacker,usertesting/webpacker,gauravtiwari/webpacker