message: Update README with GPORCA related blog posts
diff: Welcome to GPORCA, the Greenplum Next Generation Query Optimizer!
  +To understand the objectives and architecture of GPORCA please refer to the following articles:
  +* [Orca: A Modular Query Optimizer Architecture for Big Data](https://content.pivotal.io/white-papers/orca-a-modular-query-optimizer-architecture-for-big-data).
  +* [Profiling Query Compilation Time with GPORCA](http://engineering.pivotal.io/post/orca-profiling/)
  +* [Improving Constraints In ORCA](http://engineering.pivotal.io/post/making-orca-smarter/)
  +
  Want to [Contribute](#contribute)?
  GPORCA supports various build types: debug, release with debug info, release.
  