docs: add package.json to sample versionrc.js (#3)

If versionrc.js is present, package.json isn't automatically bumped unless it's included in the versionrc.json config

README.md

@@ -40,6 +40,9 @@ Here is an example configuration that updates the version, Android `versionCode`
 // .versionrc.js
 module.exports = {
   bumpFiles: [
+    {
+      filename: 'package.json',
+    },
     {
       filename: 'app.json',
       updater: require.resolve('standard-version-expo'),