Add a section to list all public instances

[youdontneedtoknow22]

I find this project the only alternative to Send from Mozilla, but we still don't have a comprehensive list of all instances like the one on the forked Send

The only instance I'm aware of is This, which is recommended on a popular german privacy blog

Adding instances will also help making the project well known, as would recommend adding it then on Privacyguides.org, but AFAIK they won't recommend one instance, but rather a list of instances like in send.