Update project documentation

Since this is the first @citusdata project I've modified very much, I thought I'd use it as an example to show the project structure we should try to use for open source projects. Important changes:

  * Moved `README` to `README.md` in order to permit richer representations
  * Made small changes to `README` for clarity/readability/correctness
  * Updated copyright information in `README` and source comments
  * Change license to LGPLv3 to allow [linking from non-GPL programs][1]
  * Added `LICENSE` file containing unmodified LGPLv3 text
  * Added `CONTRIBUTING.md` file with contribution guidelines and restrictions

@ozgune was hesitant to retrofit existing projects with this structure (low priority), but it seemed better to do so now (to facilitate discussion) rather than when we undertake a bigger OSS project.

/cc @metdos, @bredman, @sumedhpathak 

[1]: http://en.wikipedia.org/wiki/GNU_Lesser_General_Public_License#Differences_from_the_GPL