Add CHANGELOG for our next release (#798)

* Provde the changelog for this month's release.

* Add a new changelog.

* Update CHANGELOG.md

CHANGELOG.md

+ Changes by Version	
+ ==================
+ 
+ 5.0.0-alpha (2018-02-28)
+ ------------------
+ 
+ #### Agent -> Collector protocol
+ - Remove C++ keywords
+ - Move **Ref** into Span from Segment
+ - Add span type, when register an operation
+
+ #### UI -> Collector GraphQL query protocol
+ - First version protocol
+ 
+ #### Agent Changes
+ - Support gRPC 1.x plugin
+ - Support kafka 0.11 and 1.x plugin
+ - Support ServiceComb 0.x plugin
+ - Support optional plugin mechanism.
+ - Support Spring 3.x and 4.x bean annotation optional plugin
+ - Support Apache httpcomponent AsyncClient 4.x plugin 
+ - Provide automatic agent daily tests, and release reports [here](https://github.com/SkywalkingTest/agent-integration-test-report).
+ - Refactor Postgresql, Oracle, MySQL plugin for compatible.
+ - Fix jetty client 9 plugin error
+ - Fix async APIs of okhttp plugin error
+ - Fix log config didn't work
+ - Fix a class loader error in okhttp plugin
+ 
+ #### Collector Changes
+ - Support metrics analysis and aggregation for application, application instance and service in minute, hour, day and month.
+ - Support new GraphQL query protocol
+ - Support alarm
+ - Provide a prototype instrument for collector.
+ - Support node speculate in cluster and application topology. (Provider Node -> Consumer Node) -> (Provider Node -> MQ Server -> Consumer Node)
+ 
+ #### UI Changes
+ - New 5.0.0 UI!!!
+ 
+ [Issues and Pull requests](https://github.com/apache/incubator-skywalking/milestone/17)