Document lite usage

[jamesward]

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Once we ship the stub-lite with the correct deps, we should document how to use it which does kinda hint at putting build.gradle, build.gradle.kts, and pom.xml snippets on the grpc.io site or at least linking to proper samples. Note that for stub-lite there is additional config needed for protoc: https://github.com/jamesward/kotlin-samples/blob/master/run/grpc-sample-android/common/build.gradle.kts#L36-L44