Add changes file for 4.15.2

Change-Id: I691d64c4faac7158009d6f93a089cab184934893
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>

dist/changes-4.15.2.md

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+Qt Creator 4.15.2
+=================
+
+Qt Creator version 4.15.2 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 origin/v4.15.1..v4.15.2
+
+Projects
+--------
+
+### CMake
+
+* Improved performance after project load and reparse
+* Fixed crash on session switch (QTCREATORBUG-25837)
+
+### qmake
+
+* Fixed issues with executing system calls (QTCREATORBUG-25970)
+
+Test Integration
+----------------
+
+### CTest
+
+* Fixed test detection if `ctest` takes long to run (QTCREATORBUG-25851)
+
+Platforms
+---------
+
+### WASM
+
+* Fixed Python version that is on Windows (QTCREATORBUG-25897)
+
+Credits for these changes go to:
+--------------------------------
+Ahmad Samir  
+Alessandro Portale  
+Christian Stenger  
+Cristian Adam  
+Eike Ziller  
+Ivan Komissarov  
+Kai Köhne  
+Knud Dollereder  
+Michael Winkelmann  
+Mitch Curtis  
+Robert Löhning  
+Thomas Hartmann  
+Tim Blechmann  
+Tim Jenssen  
+Tuomo Pelkonen