Add editor setup instructions for Vim + vim-go (#3057)

* Add editor setup instructions for Vim + vim-go

Addresses https://github.com/bazelbuild/rules_go/issues/3056 by adding
basic instructions to get `gopls` working with autogenerated Bazel
files. Tested locally and working for protobuf as well as custom Bazel
rules + macros that generate `go_library` targets.