Update installation.md

- Fixes #303
- Due to [this rename commit](7221822) the readme was out of date
- I also took the liberty to make the installation instructions more user friendly (copy&pasteable)

docs/installation.md

@@ -9,13 +9,13 @@ want in your project. Here you can find information on how to install all the pa
 
 ## Packages
 
-Independently of which packages you will use, you will need to have the Core and Theme
-package installed.
-
-
 ```sh
+# Independently of which other packages you will use,
+# you will need to have those two packages installed:
 ember install @frontile/utilities
 ember install @frontile/theme
+
+# Optional packages:
 ember install @frontile/buttons
 ember install @frontile/collections
 ember install @frontile/status