Better documentation for setupTests.js when ejecting (#3656)

* Better documentation for setupTests.js when ejecting

When running `npm run eject` before creating `src/setupTests.js`, the resulting `package.json` file, won't contain any entry for it - and this is correct in my opinion, since otherwise Jest will crash - but it's useful to have it documented and avoid pointless waste of time.

* Added additional note about src/setupTests.js

Added another note about src/setupTests.js and `npm run eject` in Testing Components section

* Update README.md

* Update README.md

packages/react-scripts/template/README.md

@@ -1334,7 +1334,7 @@ import Adapter from 'enzyme-adapter-react-16';
 configure({ adapter: new Adapter() });
 ```
 
-(Note that **if you already ejected** before creating `src/setupTests.js`, this won’t work unless you set [this Jest option](https://facebook.github.io/jest/docs/en/configuration.html#setuptestframeworkscriptfile-string) to point to `src/setupTests.js`.)
+>Note: Keep in mind that if you decide to "eject" before creating `src/setupTests.js`, the resulting `package.json` file won't contain any reference to it. [Read here](#initializing-test-environment) to learn how to add this after ejecting.
 
 Now you can write a smoke test with it:
 
@@ -1425,7 +1425,14 @@ const localStorageMock = {
 global.localStorage = localStorageMock
 ```
 
-Note that **if you already ejected** before creating `src/setupTests.js`, this won’t work unless you set [this Jest option](https://facebook.github.io/jest/docs/en/configuration.html#setuptestframeworkscriptfile-string) to point to `src/setupTests.js`.
+>Note: Keep in mind that if you decide to "eject" before creating `src/setupTests.js`, the resulting `package.json` file won't contain any reference to it, so you should manually create the property `setupTestFrameworkScriptFile` in the configuration for Jest, something like the following:
+
+>```js
+>"jest": {
+>   // ...
+>   "setupTestFrameworkScriptFile": "<rootDir>/src/setupTests.js"
+>  }
+>  ```
 
 ### Focusing and Excluding Tests