Adjust README to support servers that are popular but not endorsed by Discord

[jonimettala]

Hello,

I'd like to know what is this repo's stance on officially endorsed but unofficial servers: Discord API and Discord Linux. And how do you want to show them here.

Unofficial officially endorsed server 101 just in case: The server is not run by Discord but it's acknowledged as an useful resource and often referred also by Discord staff. For example, DFeedback has "useful resource" links to, and only to, DAPI and DLinux.

Currently, DAPI is listed in official servers but DLinux is not listed at all. Should these both be listed as an own category below official server, titled as endorsed servers? Or right after official servers with an "unofficial" note? It'd feels too harsh to drop them under Community Resources.

[jacc]

I think we should open up servers from Servers under the official tab to a new tab called Popular Servers, as there are many servers that aren't officially endorsed but has a large community base.