Add instructions for index.html to Plugins section in documentation

[katieyang]

It may be helpful to add instructions on linking to plugins (e.g., <script src="plugins/TextLog.js"></script>)
in the index.html doc to the Plugins section of the documentation. Right now I don't think readers will know to do it unless they have some coding experience. Could be helpful for people without coding experience trying to get plugins working.

Happy to create a PR if helpful.