Update docs to link to mdempsky/gocode (currently linked nsf/gocode is no longer maintained)

[hbd]

From nsf/gocode

VERY IMPORTANT: this project is not maintained anymore, look for alternatives or forks if you need Go autocompletion tool
IMPORTANT: consider switching to https://github.com/mdempsky/gocode if you have problems starting with Go version 1.10, due to changes in binary packages architecture (introduction of package cache) I'm not going to adjust gocode for it for quite some time. There is a higher chance that fork under the given link will have some solution to the problem sooner or later.

[hbd]

Link to PR: #52

[hut8]

The fork you linked to is now also unmaintained and does not work with modules which is a showstopper for most of us - https://github.com/mdempsky/gocode

It suggests using https://github.com/stamblerre/gocode but that project is also unmaintained.

The only solution is to switch to gopls.