Plugin needs to be enclosed in ".sketchplugin" folder

[mludowise]

The installation instructions for this plugin are not entirely complete.

1. The "Download SketchingOn.zip" link doesn't actually link to a zip in the instructions https://github.com/WorkingOn/SketchingOn/releases/tag/v1.1.0
2. When downloading the source zip and unzipping it in the Sketch plugins folder, Sketch doesn't recognize it as a plugin because it doesn't have the ".sketchplugin" extension.

The easiest fix would be to enclose the "Contents" folder in an "SketchingOn.sketchplugin" folder so when users download your plugin, they can either copy the .sketchplugin file into the plugins folder or double-click on it to have Sketch install it for them. Additionally, this will be compatible with both SketchToolbox and SketchPluginManager.