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

README.md

@@ -7,11 +7,20 @@ even JavaScript Sprinkles (that all continues to live in app/assets).
 
 It's designed to work with Rails 5.1+ and makes use of the [Yarn](https://yarnpkg.com/) dependency management
 that's been made default from that version forward.
-It's also currently compatible with Rails 5.0 stable but there's absolutely no warranty
-it will still be in the future.
+
+## Installation
+
+Webpacker is currently compatible with Rails 5.0 stable, but there's no guarantee it will still be
+in the future.
+
 You can either make use of Webpacker during setup of a new application with `--webpack`
 or you can add the gem and run `bin/rails webpacker:install` in an existing application.
 
+As the rubygems version isn't promised to be kept up to date until the release of Rails 5.1, you may want to include the gem directly from GitHub:
+
+```ruby
+gem 'webpacker', github: 'rails/webpacker'
+```
 
 ## Binstubs