Update README.md

Author: nicolst

README.md

@@ -5,11 +5,15 @@ Signed JWT token exchange service
 
 ```mermaid
 sequenceDiagram
+    participant GKE
     participant Client
     participant labid
     participant Service
-    Client->>labid: Request token on /token
-    labid-->>Client: Response with JWT token
-    Client-->>Service: Request with token from labid
-    Service->>labid: Check validity of token with request to /jwks
+    GKE->>Client: Provides native JWT token
+    Client->>labid: Requests exchange of native JWT for labid token at /token
+    labid-->>GKE: Checks validity of native token against JWKS
+    labid-->>GKE: Deduces Dapla group based on Client's SA annotations
+    labid->>Client: Response with JWT token
+    Client->>Service: Request with token from labid
+    Service->>labid: Check validity of labid token against JWKS (/jwks)
 ```