docs(typescript): add triple slash directive (jest-community#340)

README.md

@@ -163,6 +163,12 @@ Since the [breaking changes]() in `25.5.0` you may also need to update your `tsc
 
 Also note that when adding this for the first time this affects which files are compiled by the TypeScript compiler and you might need to add the `include` property as well. See the [TypeScript docs](https://www.typescriptlang.org/docs/handbook/tsconfig-json.html) for more details.
 
+If the above import syntax does not work, replace it with the following:
+
+```ts
+/// <reference types="jest-extended" />           
+```  
+              
 ## Asymmetric matchers
 
 All matchers described in the API are also asymmetrical since [jest version 23](https://jestjs.io/blog/2018/05/29/jest-23-blazing-fast-delightful-testing#custom-asymmetric-matchers):