docs: add usage to README

Author: eddyerburgh

README.md

@@ -1,3 +1,21 @@
 # vue-jest
 
-Jest transform for .vue files.
+Jest Vue transformer with source map support
+
+## Usage
+
+```
+npm install --save-dev jest-vue
+```
+
+## Setup
+
+Note: this step is only required if you are using babel-jest with additional code preprocessors.
+
+To define jest-vue as a transformer for your .vue code, map .vue files to the babel-jest module.
+
+```
+"transform": {
+  ".*\\.(vue)$": "<rootDir>/node_modules/jest-vue"
+},
+```