Clarified babel-jest dependencies requirements (#285)

Fixes #273

README.md

@@ -385,7 +385,9 @@ This tells `ts-jest` (a preprocessor this preset using to transform TS files) to
 
 Some vendors publish their sources without transpiling. You need to say jest to transpile such files manually since `typescript` (and thus `ts-jest` used by this preset) do not transpile them.
 
-1. Install `@babel/preset-env` and add `babel.config.js` (or modify existing if needed) with the following content:
+1. Install dependencies required by Jest official documentation for [Babel integration](https://jest-bot.github.io/jest/docs/babel.html).
+
+2. Install `@babel/preset-env` and add `babel.config.js` (or modify existing if needed) with the following content:
 ```js
 module.exports = function(api) {
   api.cache(true);
@@ -403,7 +405,7 @@ module.exports = function(api) {
 
 *Note: do not use a `.babelrc` file otherwise the packages that you specify in the next step will not be picked up. CF [Babel documentation](https://babeljs.io/docs/en/configuration#what-s-your-use-case) and the comment `You want to compile node_modules? babel.config.js is for you!`*.
 
-2. Update Jest configuration (by default TypeScript process untranspiled JS files which is source of the problem):
+3. Update Jest configuration (by default TypeScript process untranspiled JS files which is source of the problem):
 
 ```js
 {