docs(update): add note on gulp validate task

fix #562

documentation/content/Contributing/jinja-style.md

@@ -18,3 +18,6 @@ Specifically:
 - in Jinja print statements, surround the variable with spaces inside curly braces – for example: `{{ foo.bar }}`
 - use double (`""`) quotation marks around HTML attributes
 - end files with a newline
+
+You can run `gulp validate` task to validate the templates files using [Jinja Ninja](https://github.com/ramonsaraiva/jinjaninja).
+It is not comprehensive but it does point out obvious mistakes. You can read more about using `gulp` [here]({filename}./live-reload-gulp.md).