Update README.md

[paulburlumi]

Following these instructions with the latest version of Android Studio and it complains that compile is now obsolete. True enough the gradle documentation says the same thing: https://docs.gradle.org/current/userguide/java_library_plugin.html#sec:java_library_separation

The compile configuration still exists but should not be used as it will not offer the guarantees that the api and implementation configurations provide.

[steve-perkins]

Thanks!