add alternative link to Java implementation

[fabiolimace]

Add link to another Java implementation in README file: KSUID Creator.

How to use

Create a KSUID:

Ksuid ksuid = KsuidCreator.getKsuid();

Create a KSUID with subsecond precision:

Ksuid ksuid = KsuidCreator.getKsuidMs();

Create a Monotonic KSUID, similar to Segment's sequence.go:

Ksuid ksuid = KsuidCreator.getMonotonicKsuid();

The source code has a good amount of unit tests. It also has a micro benchmark to check if the performance is good for your needs.

[fabiolimace]

Thank you!