Add note about running Jekyll with bundle exec

theme-setup/index.md

@@ -78,6 +78,22 @@ paginate: 5
 
 ---
 
+## Running Jekyll
+
+If `jekyll build` and `jekyll serve` throw errors you may have to run Jekyll with `bundled exec` instead.
+
+> In some cases, running executables without bundle exec may work, if the executable happens to be installed in your system and does not pull in any gems that conflict with your bundle.
+>
+>However, this is unreliable and is the source of considerable pain. Even if it looks like it works, it may not work in the future or on another machine.
+
+{% highlight text %}
+bundle exec jekyll build
+
+bundle exec jekyll serve
+{% endhighlight %}
+
+---
+
 ## Folder Structure
 
 {% highlight bash %}