Add changes file for Qt 5.12.0

Change-Id: I10d98d83feda0373b45c7c0911de0c2ec6ff80d4
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

dist/changes-5.12.0

+Qt 5.12 introduces many new features and improvements as well as bugfixes
+over the 5.11.x series. For more details, refer to the online documentation
+included in this distribution. The documentation is also available online:
+
+https://doc.qt.io/qt-5/index.html
+
+The Qt version 5.12 series is binary compatible with the 5.11.x series.
+Applications compiled for 5.11 will continue to run with 5.12.
+
+Some of the changes listed in this file include issue tracking numbers
+corresponding to tasks in the Qt Bug Tracker:
+
+https://bugreports.qt.io/
+
+Each of these identifiers can be entered in the bug tracker to obtain more
+information about a particular change.
+
+****************************************************************************
+*                                QWebSocket                                *
+****************************************************************************
+
+ - Added the bytesToWrite function to QWebSocket. Call this function to see
+   how many bytes are left to write.
+
+****************************************************************************
+*                             QWebSocketServer                             *
+****************************************************************************
+
+- Introduced nativeDescriptor and setNativeDescriptor. These functions are
+  replacing socketDescriptor and setSocketDescriptor which are now
+  deprecated.
+
+****************************************************************************
+*                      Platform Specific Changes                           *
+****************************************************************************
+
+Wasm
+-----
+ - [QTBUG-70339] Add support for WebSockets on Qt for WASM