[docs]: Document How to Produce Icon Fonts

[bennypowers]

Is your feature request related to a problem? Please describe.
A developer needs to extend the default Material Icons icon font. She wants to use icons from https://materialdesignicons.com/, which does not include ligatures in their published font. https://materialdesignicons.com/ has also stated that they don't intend to support their font as a first-class product going forward, so it's unlikely that ligatures will be added.

Describe the solution you'd like
Back with Polymer's iron-icon, adding an svg icon was a simple copy-paste affair, but with mwc-icon, developers will have to compile and serve their own icon fonts. I'd like for this project to document that process, so that our developer can go from no-knowledge to working icon font by following some clear steps,

Describe alternatives you've considered
A feature like iron-icon's iron-iconset-svg would also work.

Additional context
Pinging @asyncLiz

We could coordinate with the icons team and see what tools they're using to generate the fonts.

[asyncLiz]

Obsolete with M3