Docs: Add temporary notice for 4.0.0 vs 3.9.1 documentation (#2121)

Author: Lucretiel

Diff for: docs/API.md

@@ -1,3 +1,7 @@
+Note: these docs are for version v4.0.0 (aka `gulp@next`) If you're on gulp
+v3.9.1, which is the current standard `npm` release, you probably want [that
+version's documentation](https://github.com/gulpjs/gulp/blob/v3.9.1/docs/API.md).
+
 ## gulp API docs
 
 * [gulp.src](#gulpsrcglobs-options) - Emit files matching one or more globs