Add fine print explaining experimental parameters

[holly-cummins]

Resolves #55

This took a while, so doing a PR, but I think there's more to do, both visually and in terms of the information we show.
So I've raised one or two follow-on issues :D

• Add hyperlink onto repo url in fine print #62
• Use bold text for headings in fine print #63
• Show core count on fine print, even if set by a non-JVM args mechanism #64
• Show JVM args in fine print #65
• Show repo tag or hash in fine print #66

The amount of whitespace on the image is also wonky, but that needs #57.