Feature request: output binaries' --help into template

[clbarnes]

Keeping a binary crate's usage information up to date with the crate itself is really helpful. I made a very hacky python tool for piping text into markdown code blocks for this purpose, so my README would look like

# My package

## Usage

```_mybinary
```

Then do cargo run -- --help | pipe2codeblock --tgt _mybinary README.md to dump the help text into that labelled code block, and git diff --exit-code in CI to make sure every commit has it up to date.

I'd love to replace that with a template with cargo-readme: do you think that would be helpful?