Add post install instructions for icon cache

[danirabbit]

Currently we recommend folks manually refresh their icon cache:

docs/writing-apps/our-first-app/icons.md

Line 53 in 6c2feb2

If you cannot see your new icon in the Applications Menu or the Dock once you've reinstalled your app, refresh your system's icon cache using the following command:

Instead we should recommend adding the post-install command to do it automatically