Add example: CD as part of an existing project

[PeterBowman]

There are three main use cases (check README.md):

• install CD and use it via find_package
• clone and reference via COLOR_DEBUG_DIR + find_package
• embed CD into an existing project

The first two options have been covered in https://github.com/roboticslab-uc3m/color-debug/issues/9 with a new example app. I propose adding another such app to illustrate the third case.