gopls/doc: add documentation for "local" configuration

Change-Id: I9181c986b8e5aa21526ff934b62c041f3c820b7c
Reviewed-on: https://go-review.googlesource.com/c/tools/+/224117
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
diff --git a/gopls/doc/settings.md b/gopls/doc/settings.md
index e767f77..b12f8f8 100644
--- a/gopls/doc/settings.md
+++ b/gopls/doc/settings.md
@@ -56,6 +56,13 @@
 
 Default: `"pkg.go.dev"`.
 
+### **local** string
+
+This is the equivalent of the `goimports -local` flag, which puts imports beginning with this string after 3rd-party packages.
+It should be the prefix of the import path whose imports should be grouped separately.
+
+Default: `""`.
+
 ## Experimental
 
 The below settings are considered experimental. They may be deprecated or changed in the future. They are typically used to test experimental opt-in features or to disable features.