Deprecating .buckconfig.d directory inside the project

Created by: bobyangyf

Currently, buck supports a .buckconfig.d folder that contains a set of configuration files inside the project.

This support will be removed in about a weeks time. We encourage you to use "include" statements instead from the main repository .buckconfig file ("Command-line control of configuration" in https://buck.build/files-and-dirs/buckconfig.html)