Add change log for 4.2.1

Change-Id: I9161030e01bc89670da1f7122c04d1e0e230c6f3
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>

dist/changes-4.2.1.md

@@ -0,0 +1,45 @@
+Qt Creator version 4.2.1 contains bug fixes.
+
+The most important changes are listed in this document. For a complete
+list of changes, see the Git log for the Qt Creator sources that
+you can check out from the public Git repository. For example:
+
+    git clone git://code.qt.io/qt-creator/qt-creator.git
+    git log --cherry-pick --pretty=oneline v4.2.0..v4.2.1
+
+Editing
+
+* Fixed that viewport could change unexpectedly when block selection was
+  active but not visible in viewport (QTCREATORBUG-17475)
+
+All Projects
+
+* Fixed issue with upgrading tool chain settings in auto-detected kits
+
+Qbs Projects
+
+* Fixed that target OS defaulted to host OS if tool chain does not specify
+  target OS (QTCREATORBUG-17452)
+
+Generic Projects
+
+* Fixed that project files were no longer shown in project tree
+
+Debugging
+
+* Fixed issue with infinite message boxes being displayed
+  (QTCREATORBUG-16971)
+* Fixed `QObject` property extraction with namespaced Qt builds
+
+Platform Specific
+
+Windows
+
+* Fixed detection of MSVC 2017 RC as MSVC 2017
+* Fixed that environment detection could time out with MSVC
+  (QTCREATORBUG-17474)
+
+iOS
+
+* Fixed that starting applications in simulator could fail, especially with
+  iOS 10 devices (QTCREATORBUG-17336)