Add documentation

htdocs/includes/login/README.txt

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+README (english)
+---------------------------------------------
+Decription of htdocs/includes/login directory
+---------------------------------------------
+This directory contains files that handle way to validate passwords.
+If you want to add a new password checker function, just add a file in
+this directory that follow example of already existing files.
+This file must be called for example : 
+functions_mypasschecker.php
+
+Edit function name to call it:
+check_user_mypasschecker
+
+Change code of this function to return true if couple 
+$usertotest / $passwordtotest is ok for you.
+
+Then, you must edit you conf.php file to change the value of
+$dolibarr_main_authentication
+parameter to set it to :
+mypasschecker
+
+Once this is done, when you log in to Dolibarr, the function 
+check_user_mypasschecker in this file is called.
+If the function return true and login exists, login is accepted.