limit what gets published to npm

Author: softprops

package.json

@@ -30,5 +30,10 @@
   },
   "devDependencies": {
     "eslint": "^5.14.0"
-  }
+  },
+  "files": [
+    "index.js",
+    "package.json",
+    "README.md"
+  ]
 }