[ci skip] add guide for ActiveModel::SecurePassword

guides/source/active_model_basics.md

@@ -198,3 +198,49 @@ person.valid?                        # => true
 person.token = nil
 person.valid?                        # => raises ActiveModel::StrictValidationFailed
 ```
+
+### SecurePassword
+
+`ActiveModel::SecurePassword` provides a way to securely store any
+password in an encrypted form. On including this module, a
+`has_secure_password` class method is provided which defines 
+an accessor named `password` with certain validations on it.
+
+#### Requirements
+
+`ActiveModel::SecurePassword` depends on the [`bcrypt`](https://github.com/codahale/bcrypt-ruby 'BCrypt'),
+so include this gem in your Gemfile to use `ActiveModel::SecurePassword` correctly.
+In order to make this work, the model must have an accessor named `password_digest`.
+The `has_secure_password` will add the following validations on the `password` accessor:
+
+1. Password should be present.
+2. Password should be equal to its confirmation.
+3. This maximum length of a password is 72 (required by `bcrypt` on which ActiveModel::SecurePassword depends)
+
+#### Examples
+
+```ruby
+class Person
+  include ActiveModel::SecurePassword
+  has_secure_password
+  attr_accessor :password_digest
+end
+
+person = Person.new
+
+# When password is blank.
+person.valid? # => false
+
+# When the confirmation doesn't match the password.
+person.password = 'aditya'
+person.password_confirmation = 'nomatch'
+person.valid? # => false
+
+# When the length of password, exceeds 72.
+person.password = person.password_confirmation = 'a' * 100
+person.valid? # => false
+
+# When all validations are passed.
+person.password = person.password_confirmation = 'aditya'
+person.valid? # => true
+```