Add comment explaining the pkg-config file installed by IM module

src/platforminputcontextplugin/CMakeLists.txt

@@ -22,6 +22,7 @@ set_target_properties(input_plugin PROPERTIES OUTPUT_NAME plasmaimplatforminputc
 
 install(TARGETS input_plugin LIBRARY DESTINATION ${KDE_INSTALL_QTPLUGINDIR}/platforminputcontexts)
 
+# we want to share this data with some other parts of Plasma; this isn't an API for anyone to use but us
 install(DIRECTORY data/ DESTINATION ${KDE_INSTALL_INCLUDEDIR}/PlasmaKeyData)
 configure_file(plasma-key-data.pc.in plasma-key-data.pc @ONLY)
 install(FILES ${CMAKE_CURRENT_BINARY_DIR}/plasma-key-data.pc DESTINATION ${KDE_INSTALL_LIBDIR}/pkgconfig)