Add brew cask in README (#548)

Signed-off-by: makocchi-git <makocchi@gmail.com>

README.md

@@ -23,6 +23,11 @@ Lens is the only IDE you’ll ever need to take control of your Kubernetes clust
 
 Download a pre-built package from the [releases](https://github.com/lensapp/lens/releases) page. Lens can be also installed via [snapcraft](https://snapcraft.io/kontena-lens) (Linux only).
 
+Alternatively on Mac:
+```
+brew cask install lens
+```
+
 ## Development
 
 > Prerequisites: Nodejs v12, make, yarn