Added simple windows build directions

`getenv` works on Windows, removed logic to return Null when reading environment vars on Win builds.  This allows kafkacat to support the `KAFKACAT_CONFIG` environment variable.

This reverts commit d448ee77dfd5fd61c74bc8227f26e61cbaefe4f2.

Remove building private libzstd, remove --source-deps-only so librdkafka links to system's SSL libries

And let low-level consumer start at BEGINNING offset by default, not 0.

(I am the packager here fwiw)

Only print JSON-related help when JSON support is built-in

Fixes #199