Differentiate documentation between `shared` and `reference_package`.

The README docs in the `shared` repo and the `reference_package` repo overlap quite a bit. Also, we abandoned updating documentation in `reference_package` where it pertains to `shared` procedures etc., so it has gotten stale.

We need to better differentiate the documentation in `shared` from the documentation in `reference_package`. The `reference_package` documentation shouldn't really document how to use the `shared` Git submodule other than to refer the user to the `shared` documentation.

See https://github.com/crickets-and-comb/reference_package/issues/21