Add troubleshooting for Gitlab docs

Clarify that some usernames that are valid in gitlab are not
compatible with OpenFaaS Cloud due to the namespacing by username.

Signed-off-by: Alistair Hey <alistair.hey@form3.tech>

docs/GITLAB.md

@@ -129,4 +129,8 @@ Your function is now deleted.
 
 #### Delete the project
 
-If you delete your project your function will be automatically deleted from the cloud.
+If you delete your project your function will be automatically deleted from the cloud.
+
+#### Troubleshooting
+
+There are some usernames that don't conform to the buildkit and Kubernetes naming for secrets, (We use usernames for namespacing objects) - You may see `invalid reference format` in the buildkit logs - if you do there are github issues [here](https://github.com/openfaas/openfaas-cloud/issues/657) and [here](https://github.com/openfaas/openfaas-cloud/issues/644) where you can see what you need to change for things to work well with Gitlab.