Updated README to remind users to add OHAttributedLabel it to their Target Dependencies

Love OHAttributedLabel, thanks so much!

When we integrated it manually via the instructions we found that command line builds were failing on our continuous integration server. Turns out we just needed to add it as a Target Dependency so that Xcode knows to build it before your main target. This pull request is just a one line change to the README. I think it also makes it so you no longer need to explicitly go in and build OHAttributedLabel in step 2.