readme tweaks

[jasonkarns]

Leverage github's new rendering for blockquoted notes and warnings.

blockquoted notes render as:

and warnings:

AFAICT, for the nice rendering to work, it must be:

1. blockquoted (prefixed with >)
2. Note/Warning must be precisely titlecased, bold and without punctuation (colon may follow but may not be within the bold markers)

I'm ambivalent about whether the notes and warnings read better as italics or not. They were italicized before, so I left them. Though they weren't "blockquoted" before, which has its own styling that probably makes the italics unnecessary? 🤷

Also fixed up a couple broken reference links