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

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 https://github.com/webpack-contrib/webpack-bundle-analyzer/issues/367#issuecomment-658063392_