Add info about local supergraph build behaviour to cross-repo relatio…

…nships (#699)

docs/federation/cross-repo-relationships.mdx

@@ -52,6 +52,10 @@ being deployed to the final API.
 You can still easily use the Hasura DDN console to explore the supergraph and test relationships across subgraphs once
 you have created a build.
 
+If you perform a _local_ supergraph build using the CLI (ie. `ddn supergraph build local`), cross-repo relationships 
+will be ignored and will not be validated. If you run the build locally you will only see the subgraphs in that 
+repository, and any relationships to subgraphs from other repositories will be missing.
+
 ### Example
 
 Let's say you have a supergraph with two subgraphs, each managed in different repositories: `users` and `products`.