I'll let @dazuma review this, since it presumably had a background discussion I'm not aware of.

General comment though - I find AUTHENTICATION.md doc to be valuable. It serves as a good checklist for Ruby developers getting started. I use it regularly to remind myself of the auth steps. So I'd advise keeping this file.

The toplevel AUTHENTICATION.md content is basically duplicated in the AUTHENTICATION.md documentation for each individual library. We're deleting the toplevel file because it is redundant, and a bit out of date as we're updating the generated library files (see googleapis/gapic-generator-ruby#1022). We also prefer that users find their documentation through official channels (e.g. cloud.google.com or the library reference documentation on that site or on rubydoc.info) rather than from the github repository.