Docs: improve babel-jest README.md file (#14957)

Author: ShuZhong

packages/babel-jest/README.md

@@ -18,8 +18,18 @@ _Note: this step is only required if you are using `babel-jest` with additional
 
 To explicitly define `babel-jest` as a transformer for your JavaScript code, map _.js_ files to the `babel-jest` module. Typescript files are also supported.
 
+By default, it loads your existing Babel configuration (if any)
+
 ```json
 "transform": {
   "\\.[jt]sx?$": "babel-jest"
 },
 ```
+
+You can also pass further [babel options](https://babeljs.io/docs/options)
+
+```json
+"transform": {
+  "\\.[jt]sx?$": ["babel-jest", { "extends": "./babel.config.js", "plugins": ["babel-plugin-transform-import-meta"] }]
+},
+```