Clarify/update documentation on debugging plugins

[BrendaH]

On the page https://xjsframework.github.io/tutorials.html#/debugging it was not clear to me that you can only use Chrome to use the debugger.
There's a warning that "For developers using Chrome for debugging" you need to stay below version 80, but this (to me) implies that for developers using some other browser there won't be a problem. But using FireFox for example resulted in a blank page after clicking on the name of the extension I wanted to debug.
So stating more clearly, "you need Chrome/Chromium, up to version 80 for this to work" would be a good thing.

[mikeybanez]

Thanks for the comment. We'll fix this (and I think link to some good source for downloading older Chromium.)