Updated Readme

Hi. I found your code for using ElasticSearch in R. It made for a good foundation for a project that I was working on. Thought that I could give a bit back by giving some simple documentation in the readme. This should help others get up to the speed quickly with the package.

I've added some basic instructions for installing. It is really targeted at a user who has only ever installed packages directly from CRAN.

I then put a simple example for setting up a server, an index, and then making a keyword search and a query search. Nothing fancy.