Add CHANGELOG for release 0.5 (#649)

CHANGELOG/CHANGELOG-0.5.md

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+This release contains some new core features, scalability improvement and lots of stabilization improvements.
+
+Some highlights include:   
+
+ * New core features such as **etcd partitioning**, **multi-tenancy controllers & CRD**, **in-place vertical scaling of both container and VM**, **multi networking**, etc.  
+ * Verified scalability improvement of **300K pods and 10K nodes**.
+ * Stabilization, improved build&test infrastructure and more test coverage.
+
+The detailed enhancements are listed below.
+
+## Key Features and Improvements 
+
+**Unified VM/Container:** 
+
+* Add features of **in-place container vertical scaling**.
+* Add the initial implementation of **in-place VM vertical scaling**, based on vcpu and memory hotplug.
+* Bump to new libvirt version in Arktos VM runtime.
+* Enable standalone deployment of Arktos VM runtime for edge scenarios.
+* Use same cgroup hierarchy for container pods and VM pods.
+* Refactor and simplify the runtime manager code in kubelet.
+
+**Multi Tenancy:**
+
+* Add the new feature of **per-tenant CRD**. Each tenant can install their own CRDs without impacting each other.
+* Add the new feature of **tenant-shared CRD**. Tenants can share a CRD installed in system space.
+* Add support of tenant.All in client-go.
+* Add support of patch and more other commands for "--tenant" option in kubelet.
+* Update most commonly-used controllers to **tenancy-aware controllers**:
+    * Job controller
+    * Volume controller, include corresponding changes in scheduler and kubelet
+    * StatefulSet controller 
+    * Service controller
+    * Resource quota controller
+    * Daemonset controller
+    * Cronjob controller
+* Update **Tenant controller** to:
+    * Initialize default tenant role and role binding during tenant creation.
+    * Initialize default network object during tenant creation. 
+    * Support tenant deletion.
+* Stabilization and various bug fixes. 
+
+**Multi-Tenancy Networking:**
+
+* Add the new **CRD object "Network"**.
+* Initial support of **per-network service IP** allocation.
+* Add pod network annotations and check readiness in kubelet.
+* Initial implementation of flat network controller, with flannel network provider.
+
+**Scalability:**
+
+* Verified support of **300K pods with 10k nodes** within performance constraint.
+* Support **multiple etcd clusters** to shard cluster data.
+* Bump to latest etcd version and customized it for multi-etcd partitioning.
+* Stabilization and enhancement of multiple API Server partitioning.
+* Stabilization and enhancement of multiple controller partitioning.
+* Add test infrastructure support for AWS.
+* Improved build and test infrastructure.
+

CHANGELOG/README.md

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 # CHANGELOGs
 
+- [CHANGELOG-0.5.md](./CHANGELOG-0.5.md)
 - [CHANGELOG-0.2.md](./CHANGELOG-0.2.md)
-- [CHANGELOG-0.1.md](./CHANGELOG-0.1.md)
+- [CHANGELOG-0.1.md](./CHANGELOG-0.1.md)