Warn about things that we strongly recommend avoiding

#317 introduces support for a pattern we generally don't recommend: pointers as map values. This is mainly for a few legacy use cases that folks want to support, but I think we generally want to warn people that this isn't a pattern that we/the kube API conventions generally suggest/support.

We probably want something that looks a lot like our error reporting system, but works adds a "non-fatal" warning instead of a semi-fatal error. I'd expect something like:

```go
pkg.AddWarning(loader.ErrorFromNode(...))
```

and then print out warnings at the end.

#317 is the only case I know of off the top of my head, but having this infra would prob be useful in the future.

/help
/good-first-issue
/kind feature
/priority important-soon