Add readme

README.md

+Links to configure Keycloak/Nextcloud integration:
+
+- https://stackoverflow.com/questions/48400812/sso-with-saml-keycloak-and-nextcloud
+- https://stackoverflow.com/questions/51011422/is-there-a-way-to-filter-avoid-duplicate-attribute-names-in-keycloak-saml-assert