Add troubleshooting pointers for common new uses issues with adding stylesheets...

[rdwatters]

See this forum thread:

https://discuss.gohugo.io/t/having-trouble-adding-styling-to-my-hugo-site/6718

[digitalcraftsman]

We should advise users to check the developer tools of the browser tools in order to troubleshoot a problem. See https://discourse.gohugo.io/t/themes-not-showing-on-github-page/3073/4