Add format-on-save instructions (#46)

Author: pcj

docs/formatting.md

@@ -14,4 +14,12 @@ nav_order: 4
 ![1-format](https://user-images.githubusercontent.com/50580/89370237-7cc95400-d69d-11ea-8d6c-949fd099cf21.gif)
 
 To configure the version of `buildifier` that will be downloaded, configure the
-settings under `bsv.buildifier.*`.
+settings under `bsv.buildifier.*`.
+
+To format BUILD files on every save, add the following to your settings.json `> Preferences: Open Settings (JSON)`:
+
+```json
+"[bazel]": {
+    "editor.formatOnSave": true
+}
+```