Add example to be tested

Update API description.

examples/device/dfu/CMakeLists.txt

@@ -0,0 +1,41 @@
+# use directory name for project id
+get_filename_component(PROJECT ${CMAKE_CURRENT_SOURCE_DIR} NAME)
+set(PROJECT ${BOARD}-${PROJECT})
+
+# TOP is absolute path to root directory of TinyUSB git repo
+set(TOP "../../..")
+get_filename_component(TOP "${TOP}" REALPATH)
+
+# Check for -DFAMILY=
+if(FAMILY STREQUAL "rp2040")
+  cmake_minimum_required(VERSION 3.12)
+  set(PICO_SDK_PATH ${TOP}/hw/mcu/raspberrypi/pico-sdk)
+  include(${PICO_SDK_PATH}/pico_sdk_init.cmake)
+  project(${PROJECT})
+  pico_sdk_init()
+  add_executable(${PROJECT})
+
+  include(${TOP}/hw/bsp/${FAMILY}/family.cmake)
+
+  # Example source
+  target_sources(${PROJECT} PUBLIC
+    ${CMAKE_CURRENT_SOURCE_DIR}/src/main.c
+    ${CMAKE_CURRENT_SOURCE_DIR}/src/usb_descriptors.c
+  )
+
+  # Example include
+  target_include_directories(${PROJECT} PUBLIC
+    ${CMAKE_CURRENT_SOURCE_DIR}/src
+  )
+
+  # Example defines
+  target_compile_definitions(${PROJECT} PUBLIC
+    CFG_TUSB_OS=OPT_OS_PICO
+  )
+
+  target_link_libraries(${PROJECT} pico_stdlib pico_fix_rp2040_usb_device_enumeration)
+  pico_add_extra_outputs(${PROJECT})
+
+else()
+  message(FATAL_ERROR "Invalid FAMILY specified")
+endif()