GitLab: document SSH host key fingerprints

wiki/src/contribute/working_together/GitLab.mdwn

@@ -23,6 +23,14 @@ your own copy of our repositories.
 Later on, you will probably need to [[request more
 credentials|GitLab#request-access]].
 
+The first time you connect over SSH to a Git repository hosted on our GitLab,
+you will be prompted to verify the server's SSH host key. Here are the
+corresponding fingerprints:
+
+ - ECDSA: `SHA256:f/PV2zYMjEjmC2c7QLWs0gWiuadel9ZDY2a7scQ+/BE`
+ - Ed25519: `SHA256:RxjdAxT/9nqD2RKHVUxCTQeiKKWSn3bg7bjmatQO2DM`
+ - RSA: `SHA256:fXpmM5kJ9A9fHMaGUekgxXHIocTpsbmYnp/mdlhWFwQ`
+
 # Group and projects
 
 All our work is tracked in projects under the [[!tails_gitlab tails