Add mention of adblockers breaking the fetch sometimes

[ebolyen]

Current Behavior
@fedarko discovered that his adblocker prevented q2view from loading a visualization hosted on GitHub. It's unclear why an adblocker would care about a fetch to GitHub, but in any case, it does make sense that this could go wrong for other people sometimes.

Comments
A note about this would be worth adding to the about/troubleshooting section.

References
forum x-ref