docs: document --config for e2e-nightwatch

Author: yyx990803

packages/@vue/cli-plugin-e2e-nightwatch/README.md

@@ -12,6 +12,7 @@
 
   ```
   --url        run e2e tests against given url instead of auto-starting dev server
+  --config     use custom nightwatch config file (overrides internals)
   -e, --env    specify comma-delimited browser envs to run in (default: chrome)
   -t, --test   sepcify a test to run by name
   -f, --filter glob to filter tests by filename
@@ -23,6 +24,8 @@
 
 We've pre-configured Nightwatch to run with Chrome by default. If you wish to run e2e tests in additional browsers, you will need to add a `nightwatch.config.js` or `nightwatch.json` in your project root to configure additional browsers. The config will be merged into the [internal Nightwatch config](https://github.com/vuejs/vue-cli/blob/dev/packages/%40vue/cli-plugin-e2e-nightwatch/nightwatch.config.js).
 
+Alternatively, you can completely replace the internal config with a custom config file using the `--config` option.
+
 Consult Nightwatch docs for [configuration options](http://nightwatchjs.org/gettingstarted#settings-file) and how to [setup browser drivers](http://nightwatchjs.org/gettingstarted#browser-drivers-setup).
 
 ## Installing in an Already Created Project

packages/@vue/cli-plugin-e2e-nightwatch/index.js

@@ -13,6 +13,7 @@ module.exports = (api, options) => {
     usage: 'vue-cli-service e2e [options]',
     options: {
       '--url': 'run e2e tests against given url instead of auto-starting dev server',
+      '--config': 'use custom nightwatch config file (overrides internals)',
       '-e, --env': 'specify comma-delimited browser envs to run in (default: chrome)',
       '-t, --test': 'sepcify a test to run by name',
       '-f, --filter': 'glob to filter tests by filename'
@@ -41,7 +42,7 @@ module.exports = (api, options) => {
           userOptions = require(userOptionsPath)
         }
         process.env.VUE_NIGHTWATCH_USER_OPTIONS = JSON.stringify(userOptions || {})
-  
+
         rawArgs.push('--config', require.resolve('./nightwatch.config.js'))
       }