Add troubleshoot `stats: 'error-only'` to troubleshooting section

[valscion]

It would be nice if we could add this tidbit in a nicely formatted and written way to the Troubleshooting section in README.md

Check your Webpack config file, seems like having stats: 'error-only' is causing the stats.json to have very limited output.

Originally posted by @Igloczek in #367 (comment)