We should imo not support the use of gdx-pay by just downloading the jars. Developers should use Dependency Management, through Gradle, Maven or Ivy.
There is a Maven wiki page for dependencies to include. It is outdated. I'd suggest just listing the Gradle dependencies for the various libgdx submodules. Android developers using Maven are used to this, and it reduces our efforts on a new release.
Most people use Gradle to manage dependencies.
Describe in README how to use gdx-pay using managed Gradle dependencies.