Merge pull request #9050 from open-sausages/pulls/4.4/react-16-docs

DOCS: Add React 16 information

docs/en/04_Changelogs/4.4.0.md

@@ -6,6 +6,7 @@
  - [Optional migration of legacy thumbnail locations](#legacy-thumb)
  - Security patch for [SS-2018-022](https://www.silverstripe.org/download/security-releases/ss-2018-022)
  - [Correct PHP types are now returned from database queries](/developer_guides/model/sql_select#data-types)
+ - [Upgrade to React 16 in CMS](#upgrade-to-react-16-in-cms)
  - [Server Requirements](/getting_started/server_requirements/#web-server-software-requirements) have been refined:
    MySQL 5.5 end of life reached in December 2018, thus SilverStripe 4.4 requires MySQL 5.6+.  
    SilverStripe 4.3 and prior still support MySQL 5.5 for their own lifetime.
@@ -239,6 +240,11 @@ conditions.
 Yes, it will attempt to find the most recent public "hash-less" URL
 for this file and redirect to it.
 
+### Upgrade to React 16 in the CMS
+
+React-rendered UIs represent a growing proportion of the CMS, and we're continuing to invest in this technology by staying up-to-date with its newest APIs. React 16 offers a number of features that developers love, such as fragments, portals, and declarative error handling.
+
+If you have thirdparty code, such as a backend module, that depends on React UIs, your code may break due to incompatability with React 16. The most common API breakage we see is that [PropTypes has been moved to its own library](https://www.npmjs.com/package/prop-types).
 
 ### DevelopmentAdmin controllers