clarify dependency installation for contributing

CONTRIBUTING.md

@@ -10,19 +10,22 @@ Here's a quick guide for contributing:
 
 1. Fork the repo.
 
-2. Run the tests. We only take pull requests with passing tests, and it's great
+1. Make sure you have ImageMagick and Ghostscript installed. See [this section]
+(./README.md#image-processor) of the README.
+
+1. Run the tests. We only take pull requests with passing tests, and it's great
 to know that you have a clean slate: `bundle && bundle exec rake`
 
-3. Add a test for your change. Only refactoring and documentation changes
+1. Add a test for your change. Only refactoring and documentation changes
 require no new tests. If you are adding functionality or fixing a bug, we need
 a test!
 
-4. Make the test pass.
+1. Make the test pass.
 
-5. Mention how your changes affect the project to other developers and users in
+1. Mention how your changes affect the project to other developers and users in
    the `NEWS.md` file.
 
-6. Push to your fork and submit a pull request.
+1. Push to your fork and submit a pull request.
 
 At this point you're waiting on us. We like to at least comment on, if not
 accept, pull requests within seven business days (most of the work on Paperclip

README.md

@@ -944,7 +944,7 @@ guidelines:
    about writing tests for paperclip, please open a
    [GitHub issue](https://github.com/thoughtbot/paperclip/issues/new).
 
-Please see `CONTRIBUTING.md` for more details on contributing and running test.
+Please see [`CONTRIBUTING.md`](./CONTRIBUTING.md) for more details on contributing and running test.
 
 Thank you to all [the contributors](https://github.com/thoughtbot/paperclip/graphs/contributors)!
 

lib/paperclip/errors.rb

@@ -19,7 +19,9 @@ module Paperclip
     end
 
     # Will be thrown when ImageMagic cannot determine the uploaded file's
-    # metadata, usually this would mean the file is not an image.
+    # metadata, usually this would mean the file is not an image. If you are
+    # consistently receiving this error on PDFs make sure that you have
+    # installed Ghostscript.
     class NotIdentifiedByImageMagickError < Paperclip::Error
     end