[...] explanation (#1589)

Author: deemp

docs/tutorials/getting-started.md

@@ -14,7 +14,7 @@ IMPORTANT: you *must* do this or you *will* build several copies of GHC!
 
 You can configure Nix to use our binary cache, which is pushed to by CI, so should contain the artifacts that you need.
 
-You need to add the following sections to `/etc/nix/nix.conf` or, if you are a trusted user, `~/.config/nix/nix.conf` (if you don't know what a "trusted user" is, you probably want to do the former).
+You need to add the following sections to `/etc/nix/nix.conf` or, if you are a trusted user, `~/.config/nix/nix.conf` (if you don't know what a "trusted user" is, you probably want to do the former). `[...]` denote any existing entries.
 
 ```
 trusted-public-keys = [...] hydra.iohk.io:f/Ea+s+dFdN+3Y/G+FDgSq+a5NEWhJGzdjvKNGv0/EQ= [...]