Bump to v0.8 (#226)

* Bump to v0.8

* add version

docs/changelog.md

 ## Changelog
 
+### V0.8 (03/11/2020)
+
+**Highlights**
+
+- Support 4 medical dataset, UNet and CGNet.
+
+**New Features**
+
+- Support customize runner ([#118](https://github.com/open-mmlab/mmsegmentation/pull/118))
+- Support UNet ([#161](https://github.com/open-mmlab/mmsegmentation/pull/162))
+- Support CHASE_DB1, DRIVE, STARE, HRD ([#203](https://github.com/open-mmlab/mmsegmentation/pull/203))
+- Support CGNet ([#223](https://github.com/open-mmlab/mmsegmentation/pull/223))
+
 ### V0.7 (07/10/2020)
 
 **Highlights**

mmseg/version.py

 # Copyright (c) Open-MMLab. All rights reserved.
 
-__version__ = '0.7.0'
+__version__ = '0.8.0'
 
 
 def parse_version_info(version_str):