doc: add link to the Go Discord forum
I've linked the gophers discord. It's a well administered discord which already got many members, but it was never officially linked. For many people it's a quality proof if a discord is linked on the official page. I think there are much more people out there, who would prefer to use discord instead of slack or irc.
The discord already got many users without even being promoted, so it's very likely there are many people who are interested in a discord, but they don't want to use unofficial discords. This discord shouldn't be seen as a competitor for the slack, it's a platform for those, who don't want to use slack.
Change-Id: Ib1ee7173f394b810f5cccf67b498485ecbf8a7db
GitHub-Last-Rev:
286ebad994a52f55b685deeda8a86cfac4a9964b
GitHub-Pull-Request: golang/go#24176
Reviewed-on: https://go-review.googlesource.com/c/97718
Reviewed-by: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>