docs: add usage api

[yinm]

It seems there aren't references to cli.usage(text) and command.usage(text).
I think they exist in README.md is more useful, so I added.

cac/src/CAC.ts

Lines 60 to 68 in d2c6b8a

	/**
	* Add a global usage text.
	*
	* This is not used by sub-commands.
	*/
	usage(text: string) {
	this.globalCommand.usage(text)
	return this
	}

cac/src/Command.ts

Lines 58 to 61 in d2c6b8a

	usage(text: string) {
	this.usageText = text
	return this
	}

[egoist]

Can you commit the change locally or run yarn toc so that the Table of Contents gets updated?

[yinm]

@egoist
Thanks your reply.
I run yarn toc and committed b6d6e29 .

[egoist]

🎉 This PR is included in version 6.7.2 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀