Help: Add notes for topic 'labels_for_subprojects'

Help/release/dev/labels_for_subprojects.rst

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+labels_for_subprojects
+----------------------
+
+* A :variable:`CTEST_LABELS_FOR_SUBPROJECTS` CTest module variable and CTest
+  script variable was added to specify a list of labels that should be treated
+  as subprojects by CDash. To use this value in both the CTest module and the
+  ctest command line `Dashboard Client` mode (e.g. ctest -S) set it in the
+  CTestConfig.cmake config file.
+
+* A :prop_dir:`LABELS` directory property was added to specify labels
+  for all targets and tests in a directory.
+
+* A :variable:`CMAKE_DIRECTORY_LABELS` variable was added to specify
+  labels for all tests in a directory.