[ci skip] Add Ubuntu install instructions

README.md

@@ -95,6 +95,10 @@ to install GhostScript. On Mac OS X, you can also install that using Homebrew:
 
     brew install gs
     
+If you're on Ubuntu, you'll want to run the following with apt-get:
+
+    sudo apt-get install imagemagick -y
+
 ### `file`
 
 The Unix [`file` command](http://en.wikipedia.org/wiki/File_(command)) is required for content-type checking.