core-observers: create github team

This will create a core-observers GitHub team, allowing to give
permissions to them without granting individual access.

Author: pietroalbini

teams/core-observers.toml

@@ -7,6 +7,9 @@ members = [
     "skade",
 ]
 
+[github]
+orgs = ["rust-lang"]
+
 [permissions]
 bors.rust.review = true