Update INTRO-cmake.md

Use vendored libraries

Author: bozgi

docs/INTRO-cmake.md

@@ -15,6 +15,9 @@ project(hello)
 set(CMAKE_RUNTIME_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/$<CONFIGURATION>")
 set(CMAKE_LIBRARY_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/$<CONFIGURATION>")
 
+# Use vendored libs
+set(SDLTTF_VENDORED ON)
+
 # This assumes the SDL source is available in vendored/SDL
 add_subdirectory(vendored/SDL EXCLUDE_FROM_ALL)