docs(readme): add Java sample to list of signature usage code snippets

Feature Request

1. User story

As a potential API consumer, I want to see an example using Java, In order to access the signatures API quickly with that programming language.

2. Acceptance criteria

We'll be done when:

• 1. There is a Java programming language example in the "Accessing signatures with other tools and programming languages" section of the README.md document.

3. Notes

@joquijada wrote:

This section is missing a Java sample. Since Java is such a widely used programming language its absence from the README is striking. Adding a sample signature usage for Java can potentially attract more users.