Help: Add 3.15.2 release notes

Help/release/3.15.rst

@@ -362,3 +362,10 @@ Changes made since CMake 3.15.0 include the following.
   always runs in a mode aware of C++14.
 
 * Preliminary Swift support added in 3.15.0 has been updated.
+
+3.15.2
+------
+
+* In CMake 3.15.0 and 3.15.1 the :variable:`CMAKE_FIND_PACKAGE_PREFER_CONFIG`
+  variable caused the :command:`find_package` command to fail on a missing
+  package even without the ``REQUIRED`` option.  This has been fixed.