docs: Add information about .d.ts.map TypeScript declaration maps (#566)

docs/README.md

@@ -147,6 +147,16 @@ Note that `--dts` does not resolve external (aka in `node_modules`) types used i
 
 The `--dts-only` flag is the equivalent of the `emitDeclarationOnly` option in `tsc`. Using this flag will only emit the declaration file, without the JavaScript files.
 
+#### Generate TypeScript declaration maps (.d.ts.map)
+
+TypeScript declaration maps are mainly used to quickly jump to type definitions in the context of a monorepo (see [source issue](https://github.com/Microsoft/TypeScript/issues/14479) and [official documentation](https://www.typescriptlang.org/tsconfig/#declarationMap)). 
+
+They should not be included in a published NPM package and should not be confused with sourcemaps.
+
+[Tsup is not able to generate those files](https://github.com/egoist/tsup/issues/564). Instead, you should use the TypeScript compiler directly, by running the following command after the build is done: `tsc --emitDeclarationOnly --declaration`.
+
+You can combine this command with Tsup [`onSuccess`](https://tsup.egoist.sh/#onsuccess) callback.
+
 ### Generate sourcemap file
 
 ```bash