Document how to generate first version of lock file

[qdii]

Describe the bug When running bazel run @bullseye://.lock without an existing bullseye.lock.json file present, the command fails with the error:

Error in read: java.io.FileNotFoundException: /home/qdii/dev/rules_distroless/e2e/smoke/bullseye.lock.json (No such file or directory)

To Reproduce

1. Clone the e2e smoke test repository.
2. Remove or update .bzlversion to use the latest bazel version
3. Remove .bullseye.lock.json
4. Run bazel run @bullseye//:lock to regenerate it
5. Try bazel run @bullseye_nolock//:lock too

Observe the command failing

Expected behavior bullseye.lock.json is an output of the command at stage 4 or 5