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

* Add editor setup instructions for Vim + vim-go

Addresses #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.

docs/go/editors/vim.md

@@ -0,0 +1,66 @@
+# Vim + `rules_go`
+
+`rules_go` has support for integration with text editors, see [Editor and tool
+integration](https://github.com/bazelbuild/rules_go/wiki/Editor-and-tool-integration).
+This document describes integration with Vim, which will provide semantic
+autocompletion for Bazel-generated Go files, among other things.
+
+## Setup
+
+1. Install [vim-go](https://github.com/fatih/vim-go), a Vim plugin for Go
+   development with support for `gopls`.
+
+2. Follow the instructions from [Editor
+   setup](https://github.com/bazelbuild/rules_go/wiki/Editor-setup#3-editor-setup)
+   for installing `gopls` and adding a launcher script.
+   * Note that `gopls` should already be installed as part of installing `vim-go`.
+
+3. Add the following to your `.vimrc`:
+
+      ```vim
+      function! MaybeSetGoPackagesDriver()
+        " Start at the current directory and see if there's a WORKSPACE file in the
+        " current directory or any parent. If we find one, check if there's a
+        " gopackagesdriver.sh in a tools/ directory, and point our
+        " GOPACKAGESDRIVER env var at it.
+        let l:dir = getcwd()
+        while l:dir != "/"
+          if filereadable(simplify(join([l:dir, 'WORKSPACE'], '/')))
+            let l:maybe_driver_path = simplify(join([l:dir, 'tools/gopackagesdriver.sh'], '/'))
+            if filereadable(l:maybe_driver_path)
+              let $GOPACKAGESDRIVER = l:maybe_driver_path
+              break
+            end
+          end
+          let l:dir = fnamemodify(l:dir, ':h')
+        endwhile
+      endfunction
+
+      call MaybeSetGoPackagesDriver()
+
+      " See https://github.com/golang/tools/blob/master/gopls/doc/settings.md
+      let g:go_gopls_settings = {
+        \ 'build.directoryFilters': [
+          \ '-bazel-bin',
+          \ '-bazel-out',
+          \ '-bazel-testlogs',
+          \ '-bazel-mypkg',
+        \ ],
+        \ 'ui.completion.usePlaceholders': v:true,
+        \ 'ui.semanticTokens': v:true,
+        \ 'ui.codelenses': {
+          \ 'gc_details': v:false,
+          \ 'regenerate_cgo': v:false,
+          \ 'generate': v:false,
+          \ 'test': v:false,
+          \ 'tidy': v:false,
+          \ 'upgrade_dependency': v:false,
+          \ 'vendor': v:false,
+        \ },
+      \ }
+      ```
+
+    * You'll want to replace `-bazel-mypkg` with your package.
+    * If you've put your `gopackagesdriver.sh` script somewhere other than
+      `tools/gopackagesdriver.sh`, you'll need to update
+      `MaybeSetGoPackagesDriver` accordingly.