Update README.md - Testing section

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.