re #15 add two factor authentication docs

docs/helpful-guides/how-to-implement-two-factor-auth.md

@@ -0,0 +1,49 @@
+How to Implement Two-Factor Auth
+================================
+
+Two-Factor Authentication products add an additional layer of security. Typically, users are asked to prove their 
+identity by providing simple credentials such as an email address and a password. A second factor (2F) adds an extra 
+layer of unauthorized access protection by prompting the user to provide an additional means of authentication such as 
+a physical token (e.g. a card) or an additional secret that only they know.
+
+With this module is quite easy. It basically implements two-factor authentication using the following 2amigos libraries: 
+
+- [2amigos/2fa-library](https://github.com/2amigos/2fa-library)
+- [2amigos/qrcode-library](https://github.com/2amigos/qrcode-library)
+
+
+Enable Two-Factor 
+-----------------
+
+We simply need to enable two factor authentication: 
+
+```php 
+'modules' => [
+    'user' => [
+        'class' => Da\User\Module::class,
+        'enableTwoFactorAuthentication' => true
+    ]
+]
+```
+
+Now, when the user go to its settings via `user/settings`, it will display the option to enable two-factor 
+authentication or not. 
+
+When enabled, the module will show a modal with a QrCode that has to be scanned by the Google Authenticator App 
+(**Recommended**. You can download from 
+[Google Play](https://play.google.com/store/apps/details?id=com.google.android.apps.authenticator2) or 
+[iTunes](https://itunes.apple.com/us/app/google-authenticator/id388497605?mt=8)). 
+
+The application will display a code that needs to be inserted into the modal input box. If code verification goes well, 
+it will enable the two-factor for the user. 
+
+If a user has enabled the two-factor, and after successfully login via username and email, it will render a new section 
+where user will have to enter the code displayed on its Google Authenticator App in order to complete with the login 
+process. 
+
+
+### Recommended Reading
+
+- [2amigos Two-Factor Library Docs]()http://2fa-library.readthedocs.io/en/latest/)
+
+© [2amigos](http://www.2amigos.us/) 2013-2017

docs/index.md

@@ -151,6 +151,7 @@ Helpful Guides
 --------------
 
 - [How to Add Captcha Widget](helpful-guides/how-to-add-captcha-widget.md)
+- [How to Implement Two-Factor Authentication](helpful-guides/how-to-implement-two-factor-auth.md)
 - [How to Switch Identities](helpful-guides/how-to-swith-identities.md)
 - [Separate Frontend and Backend Sessions](helpful-guides/separate-frontend-and-backend-sessions.md)
 - [Social Network Authentication](helpful-guides/social-network-authentication.md)

docs/installation/configuration-options.md

@@ -3,6 +3,15 @@ Configuration Options
 
 The module comes with a set of attributes to configure. The following is the list of all available options: 
 
+#### enableTwoFactorAuthentication (type: `boolean`, default: `false`)
+
+Setting this attribute will allow users to configure their login process with two-factor authentication. 
+
+### twoFactorAuthenticationCycles (type: `integer`, default: `1`)
+
+By default, Google Authenticator App for two-factor authentication cycles in periods of 30 seconds. In order to allow 
+a bigger period so to avoid out of sync issues. 
+
 #### enableRegistration (type: `boolean`, default: `true`)
 
 Setting this attribute allows the registration process. If you set it to `false`, the module won't allow users to