Document advice about how to use config merging

[bgilbert]

We document the semantics of config merging but we don't provide any advice on how to use it. Document some examples, e.g. a hierarchical config structure with a parent config that merges a base config plus environment-specific child configs overriding pieces of the parent.

In particular, we should point out that an uncompressed child resource overriding the compressed contents of a parent should set compression to "".