MINOR: Clarify how to publish specific projects to the local repo (#11938)
MINOR: Clarify how to publish specific projects to the local repo (#11938)

The current README instruction for local publishing boils the ocean by building and installing every jar in the project with both 2.12 and 2.13. While that is some times what people want to do, they are also often trying to just build a specific jar.

Reviewers: Bill Bejeck <5c404be01b63927a28df603569da1321e84406a6@apache.org>
guozhangwang/kafka,apache/kafka,guozhangwang/kafka,guozhangwang/kafka,lindong28/kafka,lindong28/kafka,guozhangwang/kafka,apache/kafka,lindong28/kafka,apache/kafka,lindong28/kafka,apache/kafka