update readme

Author: bahmutov

README.md

@@ -23,7 +23,10 @@ Register tasks in your `cypress/plugins/index.js` file
 
 ```js
 module.exports = (on, config) => {
-  on('task', require('@cypress/code-coverage/task'))
+  require('@cypress/code-coverage/task')(on, config)
+  // IMPORTANT to return the config object
+  // with the any changed environment variables
+  return config
 }
 ```
 
@@ -109,8 +112,9 @@ Put the following in `cypress/plugins/index.js` file to use `.babelrc` file
 
 ```js
 module.exports = (on, config) => {
-  on('task', require('@cypress/code-coverage/task'))
+  require('@cypress/code-coverage/task')(on, config)
   on('file:preprocessor', require('@cypress/code-coverage/use-babelrc'))
+  return config
 }
 ```
 
@@ -122,11 +126,12 @@ If you cannot use `.babelrc` for some reason (maybe it is used by other tools?),
 
 ```js
 module.exports = (on, config) => {
-  on('task', require('@cypress/code-coverage/task'))
+  require('@cypress/code-coverage/task')(on, config)
   on(
     'file:preprocessor',
     require('@cypress/code-coverage/use-browserify-istanbul')
   )
+  return config
 }
 ```
 
@@ -349,6 +354,26 @@ npm run dev:no:coverage
 - [bahmutov/code-coverage-subfolder-example](https://github.com/bahmutov/code-coverage-subfolder-example) shows how to instrument `app` folder using `nyc instrument` as a separate step before running E2E tests
 - [bahmutov/docker-with-cypress-included-code-coverage-example](https://github.com/bahmutov/docker-with-cypress-included-code-coverage-example) runs tests inside pre-installed Cypress using [cypress/included:x.y.z](https://github.com/cypress-io/cypress-docker-images/tree/master/included) Docker image and reports code coverage.
 
+## Migrations
+
+### v2 to v3
+
+Change the plugins file `cypress/plugins/index.js`
+
+```js
+// BEFORE
+module.exports = (on, config) => {
+  on('task', require('@cypress/code-coverage/task'))
+}
+// AFTER
+module.exports = (on, config) => {
+  require('@cypress/code-coverage/task')(on, config)
+  // IMPORTANT to return the config object
+  // with the any changed environment variables
+  return config
+}
+```
+
 ## Debugging
 
 This plugin uses [debug](https://github.com/visionmedia/debug) module to output additional logging messages from its [task.js](task.js) file. This can help with debugging errors while saving code coverage or reporting. In order to see these messages, run Cypress from the terminal with environment variable `DEBUG=code-coverage`. Example using Unix syntax to set the variable: