Update README.md

README.md

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-# design-system-sketch
+# Lightning Desgn System Plugin for Sketch
+
+The Lightning Design System Plugin for Sketch surfaces SLDS components, icons, wireframes, and artboards as Sketch symbols—along with system documentation—from within many designers' favorite design tool.
+
+## Installation
+
+1. Download a free trial of Sketch if you do not have it already
+2. Download [the plugin](https://github.com/salesforce-ux/design-system-sketch/releases/latest/download/design-system-sketch.sketchplugin.zip) and unzip the file
+3. Double-click the .sketch plugin file to install it in Sketch
+4. Press ⌘J on your keyboard to open the plugin
+
+Available updates will appear at the upper right of your Sketch window.