What are the criteria for inclusion?

@hasufell wrote the following in https://github.com/haskell/error-messages/issues/540:

> The readme says:

> We are interested in building a catalog of error messages for each tool in common use (GHC, Cabal, Stack, ghcup, HLS, etc)

> But why limit to common tools? The idea of easily discoverable error codes goes beyond. Is this a concern of maintenance? Could there be tier3 supported projects that simply get assigned a namespace and can maintain it?

> As an example, cabal-cache is currently evaluating whether to follow this scheme: https://github.com/haskell-works/cabal-cache

> It's not that common (I guess), but since it's primary use case is CI, it seems very natural in terms of usability to do this. You see an error in your CI logs and can quickly jump to a page explaining it and maybe linking to workarounds.

> @newhoggy

This is a good discussion to have, so I'm moving it here so we can have it.