docs: add info for babel7 peer dep (#173)

Author: escapedcat

README.md

@@ -10,6 +10,14 @@ Jest Vue transformer with source map support
 npm install --save-dev vue-jest
 ```
 
+### Usage with Babel 7
+
+If you use [jest](https://github.com/facebook/jest) < 24.0.0 and [babel-jest](https://github.com/facebook/jest/tree/master/packages/babel-jest) make sure to install babel-core@bridge
+
+```bash
+npm install --save-dev babel-core@bridge
+```
+
 ## Setup
 
 To define `vue-jest` as a transformer for your `.vue` files, map them to the `vue-jest` module: