let me help with what I can.
DO NOT EDIT THIS FILE.
IT WILL BE REPLACED WHEN FONTCONFIG IS UPDATED.
LOCAL CHANGES BELONG IN 'local.conf'.
It means anytime you upgrade the fontconfig program the fonts.conf file will be overwrote. I know about this firsthand because the fonts.conf file that vec supplied to me that included the dirs that granny has in her fonts.conf to add was overwrote when i reinstalled fontconfig.
So if you dont plan on updating fontconfig you dont have to worry about losing your changes.
Ah! So the note isn't giving a warning that editing the file would do something bad, it's just saying that if you edit it, it'll be replaced if you update fontconfig.
Well then, what I'm doing is fine. (I thought so, as I've been doing it for a pretty long time without problems.) If I did update fontconfig and fonts.conf got overwritten, I'd simply edit the new fonts.conf to contain the fonts directories I've added to my system.
Shall I go ahead with the HowTo? The process for creating font directories and adding fonts is quite simple. I just didn't want to tell people to edit fonts.conf if that was dangerous advice.
By the way, where is "local.conf"? I couldn't find such a file.