update README file

README

@@ -47,6 +47,28 @@ to launch start with $, other lines are expected output)::
 
 The application is now usable at http://localhost:8000/
 
+Settings
+--------
+
+Settings can be passed using a local_settings.py file or the shell
+environment. Dictionaries are passed by flattening the variable name and the
+dictionnary key separated by an underscore character. For example to passe the
+variable MAIN_SITE_URL in templates, define the following environment
+variable::
+
+    export PORTAIL_CITOYEN_TEMPLATE_VARS_MAIN_SITE_URL=https://main-site.com/
+
+Those variables must be prefixed with PORTAIL_CITOYEN_ when set in a
+local_settings.py file or in the environment.
+
+Name                             Description
+================================ ============================================
+TEMPLATE_VARS                    dictionnary of variables passed to templates
+FAVICON_URL                      URL pointing to the favicon
+WCSINST_DEFAULT_VARIABLES        default value for WCSINST new instances
+PROFILE_FORM_PLUGIN_FORM_CLASS   form class to use instead of the default one
+                                 for editing user profile
+
 Migrating users from W.C.S. to portail-citoyen
 ----------------------------------------------