chore(java): add readme shell

[rhamzeh]

Description

References

Review Checklist

• I have clicked on "allow edits by maintainers".
• I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• The correct base branch is being used, if not main
• I have added tests to validate that the change in functionality is working as expected

[booniepepper]

FYI there was a weird transient failure in the tests

Run make test-integration-client-java
make build-client sdk_language=java tmpdir=/tmp/tmp.Moetv
cat: ./clients/fga-dotnet-sdk/VERSION.txt: No such file or directory

From https://github.com/openfga/sdk-generator/actions/runs/5981058069/job/16228287916?pr=193

I never saw that before. I re-ran the test and it succeeded