Create a readme section about self-hosted fonts

[LucaHermann]

Hi, it's could be relevant to add a section about self-hosted fonts. In your readme you only speak about google fonts but what about the use case we have paid somes fonts and try to use them inside MUI / Gatsby v3 ?

It could be very helpful to have the proper way to add a custom font or a premium paid font that could not be included by calling googleFont.

The few only resources available are outdated and irrelevant : dev.to medium MUI

At the end we don't know properly what have to be loaded by gatsby or MUI.

[IgorMCesar]

Did you managed to get self-hoseted fonts working?

Fonts keep FOUTing even when reloading from changes...