Include documentation in the application

doc/reverse-proxy-authentication.markdown

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+Reverse Proxy Authentication
+============================
+
+This authentication method is often used for [SSO](http://en.wikipedia.org/wiki/Single_sign-on) (Single Sign-On) especially for large organizations.
+
+The authentication is done by another system, Kanboard doesn't know your password and suppose you are already authenticated.
+
+Requirements
+------------
+
+- A well configured reverse proxy
+
+or
+
+- Apache auth on the same server
+
+
+How does this work?
+-------------------
+
+1. Your reverse proxy authenticates the user and send the username through a HTTP header.
+2. Kanboard retreive the username from the request
+    - The user is created automatically if necessary
+    - Open a new Kanboard session without any prompt assuming it's valid
+
+Installation instructions
+-------------------------
+
+### Setting up your reverse proxy
+
+This is not in the scope of this documentation.
+You should check the user login is sent by the reverse proxy using a HTTP header, and find which one.
+
+### Setting up Kanboard
+
+Create a custom `config.php` file or copy the `config.default.php` file:
+
+```php
+<?php
+
+// Enable/disable reverse proxy authentication
+define('REVERSE_PROXY_AUTH', true); // Set this value to true
+
+// The HTTP header to retrieve. If not specified, REMOTE_USER is the default
+define('REVERSE_PROXY_USER_HEADER', 'REMOTE_USER');
+
+// The default Kanboard admin for your organization.
+// Since everything should be filtered by the reverse proxy,
+// you should want to have a bootstrap admin user.
+define('REVERSE_PROXY_DEFAULT_ADMIN', 'myadmin');
+
+// The default domain to assume for the email address.
+// In case the username is not an email address, it
+// will be updated automatically as USER@mydomain.com
+define('REVERSE_PROXY_DEFAULT_DOMAIN', 'mydomain.com');
+```
+
+Notes:
+
+- If the proxy is the same web server that runs Kanboard, according the [CGI protocol](http://www.ietf.org/rfc/rfc3875) the header name will be `REMOTE_USER`. By example, Apache add `REMOTE_USER` by default if `Require valid-user` is set.
+
+- If Apache is a reverse proxy to another Apache running Kanboard, the header `REMOTE_USER` is not set (same behaviour with IIS and Nginx).
+
+- If you have a real reverse proxy, the [HTTP ICAP draft](http://tools.ietf.org/html/draft-stecher-icap-subid-00#section-3.4) proposes the header to be `X-Authenticated-User`. This de-facto standart has been adopted by a number of tools.