Emphasize ImageMagick is required for db:seed

Add comment explaining ImageMagick is required for image processing
and that db:seed will fail without it.

Author: mroderick

native-installation-instructions.md

@@ -47,7 +47,7 @@ brew install postgresql
 brew services start postgresql
 ```
 
-Install other dependencies:
+Install required dependencies:
 ```
 brew install imagemagick
 ```