Mention pwgen for generating a strong shared secret

docs/configuring-playbook-shared-secret-auth.md

@@ -11,6 +11,8 @@ matrix_synapse_ext_password_provider_shared_secret_auth_enabled: true
 matrix_synapse_ext_password_provider_shared_secret_auth_shared_secret: YOUR_SHARED_SECRET_GOES_HERE
 ```
 
+You can generate a strong shared secret with a command like this: `pwgen -s 64 1`
+
 
 ## Authenticating only using a password provider