Add missing instructions to add external types to tsconfig.json after installing the package

packages/dev-utils/README.md

@@ -7,3 +7,11 @@ Dev utils to be shared across 0x projects and packages
 ```bash
 yarn add @0xproject/dev-utils
 ```
+
+If your project is in [TypeScript](https://www.typescriptlang.org/), add the following to your `tsconfig.json`:
+
+```
+"include": [
+    "./node_modules/web3-typescript-typings/index.d.ts",
+]
+```