docs: add stdlib.md

Updates golang/go#38603

docs/stdlib.md

@@ -0,0 +1,37 @@
+# Standard library development
+
+vscode-go and gopls can be used for developing the standard library, but require configuration.
+
+First, you **must open the `src/` folder in VS Code**, not the `GOROOT`. (See [golang/go#32394](https://github.com/golang/go/issues/32394).)
+
+Then, you need to configure the workspace, by placing the following in `src/.vscode/settings.json`.
+If your Go tree is not checked out at `~/go`, you will need to change the `go.alternateTools.go` and `gopls.env.PATH` settings.
+
+```json5
+{
+    // Use the local go tool. This needs to be built with make.bash.
+    "go.alternateTools": {
+        "go": "~/go/bin/go"
+    },
+    // Build a separate set of tools. For golang/vscode-go#294.
+    "go.toolsGopath": "~/.vscode/godev",
+    // Don't reformat HTML files since we have a custom style.
+    "html.format.enable": false,
+    // Don't reorder imports since they are usually not grouped.
+    "editor.codeActionsOnSave": {
+        "source.organizeImports": false
+    },
+    "gopls": {
+        // Make tools use the local go tool. For microsoft/vscode-go#3163.
+        "env": {
+            "PATH": "$HOME/go/bin/go:$PATH",
+        },
+    },
+}
+```
+
+You can add `.vscode` to `.git/info/exclude` to avoid risking checking that file into git.
+
+If you see an "inconsistent vendoring" error, please report it at [golang/go#40250](https://github.com/golang/go/issues/40250).
+
+See also [golang/go#38603](https://github.com/golang/go/issues/38603).