Add lint script

package.json

@@ -23,7 +23,8 @@
     "start": "react-scripts start",
     "build": "react-scripts build",
     "test": "react-scripts test --env=jsdom",
-    "eject": "react-scripts eject"
+    "eject": "react-scripts eject",
+    "lint": "eslint src"
   },
   "browserslist": [
     ">0.2%",