[Feature request] Improve description of adding custom characters

[Andy]

Hi,

so I just had a lot of trouble with U+000C. It would be nice if this could be added to the defaults. But the reason I'm actually here is because I had a lot of trouble figuring out how to actually add this as a custom character. The project description mentions where to add custom characters, but it doesn't actually show how to do that. A quick little example would be enough and I think it would be very helpful. Thanks!

[Nicolas Hoizey]

fixed by #101