Add "other build systems" mention to readme

[TWiStErRob]

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.

[autonomousapps]

I know very little about Bazel and Buck and they were not inspirations for this plugin. However, if you think there is some relationship, I'd be open to you PRing a change to the README.