Add "other build systems" mention to readme

It might be worth mentioning somewhere that this Gradle plugin implements to equivalent Bazel and Buck "features": https://blog.bazel.build/2017/06/28/sjd-unused_deps.html.

This would help discoverability if someone is looking for it by those names.

Locate the repository README and review where the Gradle plugin's dependency-analysis features are described. Add a discoverability mention connecting the equivalent Bazel and Buck features, using the linked Bazel post for context. Done means the README names those build systems clearly.

Written by the indexing model from the issue text.