Clarify Sketch plugin installation

Clarify Sketch plugin installation

There was some confusion about: https://doc.qt.io/qtdesignstudio/qtbridge-sketch-setup.html#:~:text=You%20can%20use%20the%20installation,available%20at%20Plugins%20%3E%20Qt%20Bridge.

The plugin is not an executable, but it is installed by double clicking (qt-bridge.sketchplugin ).
This means the plugin is associated with Sketch and can be installed by double clicking, but is no executable.

Qt Bridge for Sketch is delivered as a Sketch plugin package.

To set up Qt Bridge for Sketch:

1. Double-click qt-bridge.sketchplugin in the sketch_bridge folder in the
installation directory of Qt Design Studio.
2. Sketch will automatically install the Sketch Bridge
3. You can launch the Sketch plugin from the Plugins file menu in Sketch.