Add links for XDG and Runcom implementations

[bkuhlmann]

Why

Hello. 👋 I've been the maintainer of the XDG and Runcom gems for ~8 years and wanted ask if it'd be possible for this project to link to these Ruby implementations so people could get started using dot-config support immediately if they wanted? The Runcom gem, especially, adheres to what is being documented here but it also provides additional XDG support for cache, data, and state in addition to config as documented in this project.