Update the `README.md` to include install instructions using bzlmod

Is your feature request related to a problem? Please describe. Update to documentation.

Describe the solution you'd like

Minimum:

Update the README.md to include information on how to load the ruleset using bzlmod.
Update the release template to include bzlmod install instructions.

Stretch

Migrate the legacy, WORKSPACE instructions to a separate doc to encourage the use of bzlmod.

Describe alternatives you've considered

N/A

Additional context

Clarify how to use the ruleset with Bazel 7.x where bzlmod will be enabled by default.

tweag / rules_sh