Write about installation

README.md

@@ -9,6 +9,10 @@ Photo by [Billy Wilson](https://www.flickr.com/photos/billy_wilson/3778943617/)
 
 # Installation
 
+- Download the latest zip file on https://github.com/tomorrowkey/sketch3-mosaic-plugin/releases
+- Unzip it
+- Open sketch3-mosaic.sketchplugin to install
+
 # Usage
 
 Plugins > Mosaic > Apply mosaic