Add 0.5 release notes. (#2365)

* Add 0.5 release notes.

* Add note.

* Add note about Modin.

* Add vectorized environments note.

* Link to PR instead of commit.

* Add autoscaler link.

* syntax

site/_posts/2018-07-06-ray-0.5-release.markdown

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+---
+layout: post
+title: "Ray: 0.5 Release"
+excerpt: "This post announces the release of Ray 0.5."
+date: 2018-07-06 14:00:00
+---
+
+We are pleased to announce the 0.5 release of [Ray][1]. To upgrade to the latest
+version, run
+
+```
+pip install -U ray
+```
+
+## Refactoring
+
+One primary development is a substantial refactoring of the backend, which is
+currently off by default, but can be turned on by starting Ray with the
+environment variable `RAY_USE_XRAY=1`. This will be turned on soon after more
+testing and development and will be on by default in the next release. Some
+highlights of the refactor include the following:
+- Merging the "local scheduler" and the "plasma manager" into a single "raylet"
+  process (consisting of a "node manager" and an "object manager").
+- Faster object transfers by using multiple threads to send objects in chunks.
+- The introduction of a "lineage cache" to improve latency by removing the
+  global control store from the scheduling critical path.
+- The removal of the global scheduler (which is replaced by direct node-manager
+  to node-manager communication)
+- Providing a standardized table-based interface to the Redis-backed global
+  control store.
+- A preliminary approach for automatically flushing global control store
+  metadata to avoid out-of-memory errors for long-running jobs.
+Note that the refactoring is currently under development and does not yet
+support fault tolerance.
+
+## RLlib Improvements
+
+A number of improvements have been made to [RLlib][2]:
+- Added DDPG [[1877]](https://github.com/ray-project/ray/pull/1877).
+- Added a policy server example [[2232]](https://github.com/ray-project/ray/pull/2232).
+- Multi-agent support [[2286]](https://github.com/ray-project/ray/pull/2286).
+- Modular policy graphs [[2294]](https://github.com/ray-project/ray/pull/2294).
+- RNN support [[2299]](https://github.com/ray-project/ray/pull/2299).
+- Vectorized environments [[2170]](https://github.com/ray-project/ray/pull/2170).
+- Many bug fixes.
+
+## Tune
+
+A number of improvements have been made to [Tune][3]:
+- HyperOpt integration [[1763]](https://github.com/ray-project/ray/pull/1763).
+- Many bug fixes and improvements.
+
+## Pandas on Ray
+
+Pandas on Ray has moved to the [Modin][5] project and will continue to be
+developed there.
+
+## Autoscaler
+
+The [autoscaler][6] has grown more mature and now supports Google Cloud
+Platform:
+- Support for GCP [[2061]](https://github.com/ray-project/ray/pull/2061).
+- Many bug fixes and additional features.
+
+## Java
+
+Ray now supports Java thanks to contributions from [Ant Financial][4]:
+- Support for Java (e.g., [[2092]](https://github.com/ray-project/ray/pull/2092) [[2094]](https://github.com/ray-project/ray/pull/2094) [[2151]](https://github.com/ray-project/ray/pull/2151) [[2171]](https://github.com/ray-project/ray/pull/2171) [[2225]](https://github.com/ray-project/ray/pull/2225) [[2245]](https://github.com/ray-project/ray/pull/2245)).
+
+## Other Features
+
+- A named actors prototype [[2129]](https://github.com/ray-project/ray/pull/2129).
+- Usage of airspeed velocity for regression testing [[2113]](https://github.com/ray-project/ray/pull/2113).
+- More efficient serialization of numpy arrays [[1889]](https://github.com/ray-project/ray/pull/1889).
+
+
+[1]: https://github.com/ray-project/ray
+[2]: http://ray.readthedocs.io/en/latest/rllib.html
+[3]: http://ray.readthedocs.io/en/latest/tune.html
+[4]: https://www.antfin.com/
+[5]: https://github.com/modin-project/modin
+[6]: http://ray.readthedocs.io/en/latest/autoscaling.html