Update docs regarding mongoid, similar to rails3-mongoid-omniauth

[MarkDBlackwell]

For whoever must use MongoDB versions earlier than 2.2.0, please update these docs (README and tutorial section 'Installing MongoDB') similar to your comment and pull request for Rails Example App rails3-mongoid-omniauth.

On the Debian GNU/Linux operating system, as of March 2013, the latest stable version is MongoDB 2.0.0. With MongoDB 2.0.0, the Mongoid gem must be version 3.0.x. See the "Mongoid installation instructions":http://mongoid.org/en/mongoid/docs/installation.html#installation. Change your Gemfile to use an earlier Mongoid version:
+
+<pre>
+gem 'mongoid', '~> 3.0.1'
+</pre>