Fix#3905 by removing emoji (and move the comment into the if block for less ambiguity).
Adds `latexmk` to README.
Note that latexpdf doesn't seem to like `cmd:ref` links, possibly because the
reference location is inside a latex comment block, but I was under the
impression that there was a reference location in there previously which was
working fine. May be related to how the `cmd:def` block expands (or doesn't as
the case may be).
