Allow running eslint with npm run eslint

Document this command in README.md file

README.md

@@ -33,6 +33,19 @@ ln -s `pwd`/dist ~/.local/share/cockpit/starter-kit
 After changing the code and running `make` again, reload the Cockpit page in
 your browser.
 
+# Running eslint
+
+This plugin uses [ESLint](https://eslint.org/) to automatically check
+JavaScript code style in `.jsx` and `.es6` files.
+
+The linter is executed within every build as a webpack preloader.
+
+For developer convenience, the ESLint can be started explicitly by:
+
+    $ npm run eslint
+
+Rules configuration can be found in the `.eslintrc.json` file.
+
 # Automated Testing
 
 Run `make check` to build an RPM, install it into a standard Cockpit test VM

package.json

@@ -7,7 +7,8 @@
   "author": "",
   "license": "LGPL-2.1",
   "scripts": {
-    "build": "webpack"
+    "build": "webpack",
+    "eslint": "eslint --ext .jsx --ext .es6 ."
   },
   "devDependencies": {
     "@babel/core": "^7.0.0",