docs: add information about JavaScript API (#1037)

Author: ocavue

docs/README.md

@@ -195,7 +195,7 @@ If you have multiple entry files, each entry will get a corresponding `.d.ts` fi
 
 Note that `--dts` does not resolve external (aka in `node_modules`) types used in the `.d.ts` file, if that's somehow a requirement, try the experimental `--dts-resolve` flag instead.
 
-Since tsup version 7.4.0, you can also use `--experimental-dts` flag to generate declaration files. This flag use [@microsoft/api-extractor](https://www.npmjs.com/package/@microsoft/api-extractor) to generate declaration files, which is more reliable than the previous `--dts` flag. It's still experimental and we are looking for feedbacks. 
+Since tsup version 7.4.0, you can also use `--experimental-dts` flag to generate declaration files. This flag use [@microsoft/api-extractor](https://www.npmjs.com/package/@microsoft/api-extractor) to generate declaration files, which is more reliable than the previous `--dts` flag. It's still experimental and we are looking for feedbacks.
 
 To use `--experimental-dts`, you would need to install `@microsoft/api-extractor`, as it's a peer dependency of tsup:
 
@@ -581,6 +581,22 @@ Use `--publicDir` flag to copy files inside `./public` folder to the output dire
 
 You can also specify a custom directory using `--publicDir another-directory`.
 
+### JavaScript API
+
+If you want to use `tsup` in your Node.js program, you can use the JavaScript API:
+
+```js
+import { build } from 'tsup'
+
+await build({
+  entry: ['src/index.ts'],
+  sourcemap: true,
+  dts: true,
+})
+```
+
+For all available options for the `build` function, please see [the API docs](https://paka.dev/npm/tsup).
+
 ## Troubleshooting
 
 ### error: No matching export in "xxx.ts" for import "xxx"