Cause the cmake include dirs to also have ${CMAKE_INSTALL_INCLUDEDIR}/json-c, so downstream packages that use cmake to link against json-c can choose whether to include headers as just e.g. #include <json_object.h>, if they care to do so.

Update the README to better explain this, and make a few other tweaks.

CMakeLists.txt

@@ -503,7 +503,7 @@ install(TARGETS ${CMAKE_TARGETS}
     RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
     LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
     ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
-    INCLUDES DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}
+    INCLUDES DESTINATION ${CMAKE_INSTALL_INCLUDEDIR} ${CMAKE_INSTALL_INCLUDEDIR}/json-c
 )
 
 install(EXPORT ${PROJECT_NAME}-targets