Add changlog. (#2171)

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Release Notes.

6.0.0-GA

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#### Java Agent

- Support gson plugin(optional).

- Support canal plugin.

- Fix missing ojdbc component id.

- Fix dubbo plugin conflict.

- Fix OpenTracing tag match bug.

- Fix a missing check in ignore plugin.

#### Backend

- Adjust service inventory entity, to add properties.

- Adjust service instance inventory entity, to add properties.

- Add nodeType to service inventory entity.

- Fix when operation name of local and exit spans in ref, the segment lost.

- Fix the index names don't show right in logs. 

- Fix wrong alarm text.

- Add test case for span limit mechanism.

- Add telemetry module and prometheus implementation, with grafana setting.

- A refactor for register API in storage module.

#### UI

- Add new trace view.

- Add word-break to tag value. 

#### Document

- Add two startup modes document.

- Add PHP agent links.

- Add some cn documents.

- Update year to 2019

- User wall updated.

- Fix a wrong description in `how-to-build` doc.

All issues and pull requests are [here](https://github.com/apache/incubator-skywalking/milestone/30?closed=1)

6.0.0-beta

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