Is your feature request related to a problem? Please describe.
Provide help in CLI for sub-commands

Describe the solution you'd like
Sub-commands help should describe the sub-command's functionality and options available for the sub-command

Describe alternatives you've considered
N/A

Additional context
See: golang-migrate/migrate#370

Currently, the README.md file lists all tables with their full table comment.

I'm putting a lot of info in some of my table descriptions, including SQL code blocks. These render rather badly in the comment column of the central Tables table, which treats markdown just a little different.

Perhaps the Tables table should only use the first paragraph of each table description.

E.g.

This repo now has awesome code search support at: https://cs.opensource.google/google-cloud-cpp, but there is no support for cross-reference linking. We can enable that, but it requires us to set up a new build that produces the necessary kythe artifacts, and upload them to a GCS bucket, and a few more configuration steps. The details are described in an internal document at: go/google-cloud-cpp:k

Looks like Google recently released a cloud spanner emulator - It's probably worth testing against this locally and then updating our README with instructions on how specifically to use it with this service.

What

Package document comment is missing.

It is useful to understand how to use the packages.