docs: add release date

close #28649

CHANGELOG.en-US.md

@@ -17,6 +17,8 @@ timeline: true
 
 ## 4.9.4
 
+`2020-12-16`
+
 - 🐞 Fix Menu delayed hover color transition in Chrome. [#28372](https://github.com/ant-design/ant-design/pull/28372)
 - 🐞 Fix Tree node connection line position problem. [#28354](https://github.com/ant-design/ant-design/pull/28354) [@maksnester](https://github.com/maksnester)
 - 💄 Fix Table fixed column `z-index` which makes Dropdown not showing on top. [#28346](https://github.com/ant-design/ant-design/pull/28346)

CHANGELOG.zh-CN.md

@@ -17,6 +17,8 @@ timeline: true
 
 ## 4.9.4
 
+`2020-12-16`
+
 - 🐞 修复 Menu 在 Chrome 下 hover 色延迟变化的问题。[#28372](https://github.com/ant-design/ant-design/pull/28372)
 - 🐞 修复 Tree 的节点连接线在滚动时位置偏移的问题。[#28354](https://github.com/ant-design/ant-design/pull/28354) [@maksnester](https://github.com/maksnester)
 - 💄 修复 Table 固定列 `z-index` 样式让 Dropdown 无法展示在最顶层的问题。[#28346](https://github.com/ant-design/ant-design/pull/28346)