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

packages/0x.js/README.md

@@ -18,6 +18,15 @@ npm install 0x.js --save
 import { ZeroEx } from '0x.js';
 ```
 
+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",
+    "./node_modules/ethers-typescript-typings/index.d.ts"
+]
+```
+
 #### UMD:
 
 **Install**