Documented devkit requirement for Windows

https://github.com/thoughtbot/paperclip/issues/1543

README.md

@@ -57,7 +57,7 @@ GhostScript to be installed. On Mac OS X, you can also install that using Homebr
 
 
 ### `file` command
 ### `file` command
 
 
-The Unix [`file` command](http://en.wikipedia.org/wiki/File_(command)) is required for content type checking.
+The Unix [`file` command](http://en.wikipedia.org/wiki/File_(command)) is required for content type checking. This utility isn't available in Windows, but comes bundled with Ruby Devkit. So Windows users must make sure that the devkit is installed and added to system `PATH`. This dependency is being reviewed and other cross-platform solutions may be opted in future releases.
 
 
 Installation
 Installation
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