docs: add README help/usage output (#93)

README.md

@@ -20,3 +20,27 @@
     <img src="https://api.securityscorecards.dev/projects/github.com/nodejs/api-docs-tooling/badge" alt="api-docs-tooling scorecard badge" />
   </a>
 </p>
+
+## Usage
+
+Local invocation:
+
+```sh
+$ npx api-docs-tooling --help
+```
+
+```sh
+Usage: api-docs-tooling [options]
+
+CLI tool to generate API documentation of a Node.js project.
+
+Options:
+  -i, --input [patterns...]  Specify input file patterns using glob syntax
+  -o, --output <path>        Specify the relative or absolute output directory
+  -v, --version <semver>     Specify the target version of Node.js, semver compliant (default: "v22.6.0")
+  -c, --changelog <url>      Specify the path (file: or https://) to the CHANGELOG.md file (default:
+                             "https://raw.githubusercontent.com/nodejs/node/HEAD/CHANGELOG.md")
+  -t, --target [mode...]     Set the processing target modes (choices: "json-simple", "legacy-html",
+                             "legacy-html-all")
+  -h, --help                 display help for command
+```