Add 16.3.2 changelog (facebook#12621)

CHANGELOG.md

@@ -6,6 +6,24 @@
   </summary>
 </details>
 
+## 16.3.2 (April 16, 2018)
+
+### React
+
+* Improve the error message when passing `null` or `undefined` to `React.cloneElement`. ([@nicolevy](https://github.com/nicolevy) in [#12534](https://github.com/facebook/react/pull/12534))
+
+### React DOM
+
+* Fix an IE crash in development when using `<StrictMode>`. ([@bvaughn](https://github.com/bvaughn) in [#12546](https://github.com/facebook/react/pull/12546))
+* Fix labels in User Timing measurements for new component types. ([@bvaughn](https://github.com/bvaughn) in [#12609](https://github.com/facebook/react/pull/12609))
+* Improve the warning about wrong component type casing. ([@nicolevy](https://github.com/nicolevy) in [#12533](https://github.com/facebook/react/pull/12533))
+* Improve general performance in development mode. ([@gaearon](https://github.com/gaearon) in [#12537](https://github.com/facebook/react/pull/12537))
+* Improve performance of the experimental `unstable_observedBits` API with nesting. ([@gaearon](https://github.com/gaearon) in [#12543](https://github.com/facebook/react/pull/12543))
+
+### React Test Renderer
+
+* Add a UMD build. ([@bvaughn](https://github.com/bvaughn) in [#12594](https://github.com/facebook/react/pull/12594))
+
 ## 16.3.1 (April 3, 2018)
 
 ### React