Help: Add FindGTest-AUTO-SOURCES topic release notes

Help/release/dev/FindGTest-AUTO-SOURCES.rst

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+FindGTest-AUTO-SOURCES
+----------------------
+
+* The :module:`FindGTest` module ``gtest_add_tests`` macro learned
+  a new ``AUTO`` option to automatically read the :prop_tgt:`SOURCES`
+  target property of the test executable and scan the source files
+  for tests to be added.