Font Metrics explainer notes

Hi all,

I agreed during the F2F to write a basic explainer for the font metrics
area.

There was one already there! I added some basic additional stuff to it
though:
https://github.com/w3c/css-houdini-drafts/blob/master/font-metrics-api/README.md.
I'd encourage anyone interested in this area to check out what's been
written down and contribute to the discussion, either directly in the
explainer or via issues filed with the "Font Metrics" label.

If you think I haven't captured the spirit of our F2F conversation
correctly, or if there are important details I've left out, please let me
know and I'll add extra stuff.

Thanks,
    -Shane