Document how to cleanup after test runs

Adding an example for using Capybara/FactoryGirl to test attachments. I
ran into an issue on my project where the files I was uploading in my
test were getting saved multiple times, leading to hundreds of extra
files before I caught it. I did a bunch of research and found this to be
the most elegant solution (saving files in the testing environment to a
custom location and then deleting that directory after the suite runs).
I thought some clarification in the README would help people deal with
the more easily in the future.

README.md

@@ -783,6 +783,25 @@ end
 The important part here being the inclusion of `ENV['TEST_ENV_NUMBER']`, or the
 similar mechanism for whichever parallel testing library you use.
 
+**Integration Tests**
+
+Using integration tests with FactoryGirl may save multiple copies of
+your test files within the app. To avoid this, specify a custom path in
+the `config/environments/test.rb` like so:
+
+```ruby
+Paperclip::Attachment.default_options[:path] = "#{Rails.root}/spec/test_files/:class/:id_partition/:style.:extension"
+```
+
+Then, make sure to delete that directory after the test suite runs by adding
+this to `spec_helper.rb`.
+
+```ruby
+config.after(:suite) do
+  FileUtils.rm_rf(Dir["#{Rails.root}/spec/test_files/"])
+end
+```
+
 Contributing
 ------------