adding gemfile and changing description to bundle

Gemfile

+source 'https://rubygems.org'
+gem 'github-pages', group: :jekyll_plugins

README.md

@@ -66,19 +66,20 @@ you with the exact same versions used by GitHub to compile your site.
 If you already have Ruby, the install part should be as easy as
 
 ~~~
-gem install github-pages
+gem install bundler
+bundle install
 ~~~
 
-Note that you will need Ruby headers (`ruby-dev` package on Ubuntu) in
-order to compile C dependencies.
+in the local clone of this repository. 
 
-On OS X, you can just type `sudo gem install github-pages`.
+ Note that you will need Ruby headers (`ruby-dev` package on Ubuntu) in
+order to compile C dependencies. On OS X, just prefix these lines by a sudo.
 
 Now you can `cd` into your local clone of the repository and launch
 the compilation by
 
 ~~~
-jekyll serve -w -b''
+bundle exec jekyll serve
 ~~~
 
 Your site will be generated in a `_site` sub-directory, and served