Create readme.md

Signed-off-by: mzico <mohib@gluu.org>

Server/integrations/casa_external_user/readme.md

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+# CASA with external user [ Work in progress ] 
+
+This script integrate basic_multi_auth ( https://github.com/GluuFederation/oxAuth/tree/master/Server/integrations/basic.multi_auth_conf ) with CASA which allows organization to allow their external users ( who are being pulled or pushed from remote AD or LDAP ) to use CASA. 
+
+## How to use this script: 
+
+ - This feature is only available from 4.5 and above.
+ - We need new CASA war to use this script for now:
+   - Download war: https://maven.gluu.org/maven/org/gluu/casa/4.5.4.Final/casa-4.5.4.Final.war
+   - Update your server with this war. 
+ - Use attached script in Person Authentication Script.