Please add example/sample code to README.md

Hi there,

it looks like you're working hard on something similar to python's fonttools library, only based on rust. This looks very promising and I am very curious to play with it. Would it be possible to get some sample code on how to load and modify a font.
I was thinking about something like: https://github.com/foliojs/fontkit#example

Thanks a lot in advance, Olli

Start by reviewing README.md and the existing Rust API for loading and modifying fonts. Add a concise sample showing both operations, based on the style of the linked fontkit example, and verify that it matches the current API and is understandable to a new user.

Written by the indexing model from the issue text.