gulp: add README (#7200)

plugins/gulp/README.md

+# gulp plugin
+
+This plugin adds autocompletion for your [`gulp`](https://gulpjs.com/) tasks. It grabs all available tasks from the `gulpfile.js` in the current directory.
+
+To use it, add `gulp` to the plugins array of your `.zshrc` file:
+```
+plugins=(... gulp)
+```