Add two factor authentication

docs/2fa.markdown

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+Two factor authentication
+=========================
+
+Each user can enable the [two factor authentication](http://en.wikipedia.org/wiki/Two_factor_authentication).
+After a successful login, a one-time code (6 characters) is asked to the user to allow the access to Kanboard.
+
+This code have to be provided by a compatible software generally installed on your smartphone.
+
+Kanboard use the [Time-based One-time Password Algorithm](http://en.wikipedia.org/wiki/Time-based_One-time_Password_Algorithm) defined in the [RFC 6238](http://tools.ietf.org/html/rfc6238).
+
+There are many software compatible with the standard TOTP system.
+By example, you can use these free and open source applications:
+
+- [Google Authenticator](https://github.com/google/google-authenticator/) (Android, iOS, Blackberry)
+- [FreeOTP](https://fedorahosted.org/freeotp/) (Android, iOS)
+- [OATH Toolkit](http://www.nongnu.org/oath-toolkit/) (Command line utility on Unix/Linux)
+
+This system can work offline and you don't necessary need to have a mobile phone.
+
+Setup
+-----
+
+1. Go to your user profile
+2. On the left, click on **Two factor authentication** and check the box
+3. A secret key is generated for you
+
+![2FA](http://kanboard.net/screenshots/documentation/2fa.png)
+
+- You have to save the secret key in your TOTP software. If you use a smartphone, the easiest solution is to scan the QR code with FreeOTP or Google Authenticator.
+- Each time you will open a new session, a new code will be asked
+- Don't forget to test your device before closing your session
+
+A new secret key is generated each time you enable/disable this feature.