Add docs for testing SSO

.env.example

@@ -0,0 +1 @@
+OIDC_CLIENT_SECRET=

README.md

@@ -1,2 +1,18 @@
 # mitgliederverwaltung
 
+## Testing SSO with rauthy
+
+1. `just run`
+1. go to [localhost:8080], go to the Admin area
+1. Login with "admin@localhost" and password from `dev/rauthy/config.toml`
+1. add client from the admin panel
+    - Client ID: mv
+    - redirect uris: http://localhost:4000/auth/user/rauthy/callback
+    - Authorization Flows: authorization_code, password, client_credentials (maybe also just fewer needed)
+    - allowed origins: http://localhost:4000
+    - access/id token algortihm: RS256 (EDDSA did not work for me, found just few infos in the ashauthentication docs)
+1. copy client secret to `.env` file
+1. abort and run mix phx.server again
+
+
+