Add a Help menuitem (#180)

added a new link on the content.html page and updated the corresponding action in the main.js file
> So you want to increase the height of it, so it can fit all of the entries without showing this scrollbar.

@whimboo , what about suppressing those optional  [options][1]?
I hope it would be autosized. I'll try that out, and leave a feedback.

[1]: https://developer.mozilla.org/Add-ons/SDK/High-Level_APIs/panel#Panel%28options%29 "Panel options"