Make it possible to add a README to the inventory

[PeterJCLaw]

Currently the inventory tooling emits errors if there are files other than assets in the repo. While this is reasonable in the main, it means that newcomers struggle to get going when using the inventory.

I suggest that we want to allow some very specific paths to be excepted from the asset name validation, in the same way that /.meta is.

This probably only means adding /README.md to the list of exceptions as I can't immediately think of any other files we'd want to exclude.