docs: add changeog

Author: QingWei-Li

Diff for: CHANGELOG.md

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+
+3.0.0 / 2017-02-19
+==================
+
+#### Breaking change
+- Not support IE9
+- The route becomes: `/#/page?id=slug`. The previous route will be redirected
+
+#### Features
+- Faster rendering
+- Clearer source code
+- More powerful API
+- Auto header for each page [#78](https://github.com/QingWei-Li/docsify/issues/78)
+- Localization for search plugin [#80](https://github.com/QingWei-Li/docsify/issues/80)
+- Execute the script in markdown (see `executeScript`)