Update Readme

* Explain what the score does
* Add in Gem version badge
* formatting

README.md

@@ -1,8 +1,12 @@
 # devise_zxcvbn
 
+[![Gem Version](https://badge.fury.io/rb/devise_zxcvbn.png)](http://badge.fury.io/rb/devise_zxcvbn)
+
 Plugin for devise to reject weak passwords, using [zxcvbn-ruby](https://github.com/envato/zxcvbn-ruby) which is a ruby port of [zxcvbn: realistic password strength estimation](https://tech.dropbox.com/2012/04/zxcvbn-realistic-password-strength-estimation/). 
 The user's password will be rejected if the score is below 4 by default. It also uses the email as user input to zxcvbn, to downscore passwords containing the email.
 
+The scores 0, 1, 2, 3 or 4 are given when the estimated crack time (seconds) is less than 10**2, 10**4, 10**6, 10**8, Infinity.
+
 ## Installation
 
 Add this line to your application's Gemfile:
@@ -16,14 +20,18 @@ Add this line to your application's Gemfile:
       devise :database_authenticatable, :zxcvbnable
     end
 
-Default parameters
+### Default parameters
+
+A score of less than 3 is not recommended.
 
     Devise.setup do |config|
-      config.min_password_score = 4  # 0, 1, 2, 3 or 4
+      config.min_password_score = 4
     end
 
 ### Error Message
 
+Example error message, the `score` and `min_password_score` variables are also passed through if you need them.
+
     # config/locale/devise.en.yml
     en:
       errors: