add Rbenv and RVM notes

[songhuangcn]

Add Rbenv and RVM notes into Getting_started Doc.

Details

I deploy my project with Mina step by step according to the document 'getting_started.md', then I encountered an error:

bash: line 82 bundle: command not found

At last, I found this is caused by my Rbenv environment, I didn't take note of the some comment in deploy.rb

...
require 'mina/bundler'
require 'mina/rails'
require 'mina/git'
# require 'mina/rbenv'  # for rbenv support. (http://rbenv.org)
# require 'mina/rvm'    # for rvm support. (http://rvm.io)
...

...
task :environment do
  # If you're using rbenv, use this to load the rbenv environment.
  # Be sure to commit your .ruby-version or .rbenv-version to your repository.
  # invoke :'rbenv:load'

  # For those using RVM, use this to load an RVM version@gemset.
  # invoke :'rvm:use[ruby-1.9.3-p125@default]'
end
...

The Ruby Version Tool(Rbenv or RVM) should be used by most people, so, we may add some notes into the Doc 'getting_started.md' to remind user!

[devvmh]

Better grammar:

You may be using a ruby versioning tool (rbenv or RVM( to manage ruby and gems. If you are using one of them, don't forget to uncomment these settings: