Adding core.rrb-vector paper and a Clojure cross reference README.

Adding the core.rrb-vector paper. This core library is used extensively in Instaparse. 

I also created a Clojure cross reference README under the Clojure directory. It would be helpful if we could cross reference any of the other papers related to Clojure in the repo in clojure/README.md. We should be doing this for other programming languages and topics as well.