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
  * Change license to LGPLv3 to allow [linking from non-GPL programs][1]
  * Added `LICENSE` file