readme.md missing documentation for Kotlin-DSL of Gradle

[mattdkerr]

Please answer these questions before submitting your issue.

What version of gRPC are you using?

This is in reference to the current readme.md

What did you expect to see?

Kotlin-DSL example of how to use the Gradle build extension, now that it is 1.0 with the 5.0 release of Gradle. For documentation: https://blog.gradle.org/groovy-kotlin-dsl-samples

[carl-mastrangelo]

@mattdkerr We don't have a lot of expertise on Kotlin (our expert changed teams 😢 ). I think this is a "patches welcome" scenario.

[mattdkerr]

@carl-mastrangelo I have a fork with a sample of how this could work: https://github.com/mattdkerr/grpc-java/blob/master/README.md
The code is used in a proof of concept project I am working on currently, so I've verified it as working. There's a comment in there that could be removed, but it's in reference to a different issue for Kotlin-DSL that's also filed.
Diff is here: https://github.com/mattdkerr/grpc-java/commit/f5c506acc41e3a3db70bb62febac4518124e0178#diff-04c6e90faac2675aa89e2176d2eec7d8
(made one more commit after this to add a missing copy/paste line for $kotlinVersion to resolve)

[srini100]

@mattdkerr, thanks for the Kotlin example. Since we don't have much expertise in Kotlin, we could accept your PR as a community maintained option i.e., have a comment like "Kotlin (community supported)" in the example.

[mattdkerr]

I have created a Pull Request from my fork after revising it. Please take a look.