[ci skip] Merge ActiveModel::SecurePassword guide from @aditya-kapoor

Merge branch 'active-model-secure-password-guide' of https://github.com/aditya-kapoor/rails into aditya-kapoor-active-model-secure-password-guide

Conflicts:
	guides/source/active_model_basics.md

guides/source/active_model_basics.md

@@ -506,3 +506,49 @@ Finished in 0.024899s, 240.9735 runs/s, 1204.8677 assertions/s.
 An object is not required to implement all APIs in order to work with
 Action Pack. This module only intends to provide guidance in case you want all
 features out of the box.
+
+### 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
+```