Document how to switch to a C++ Project

[johnthagen]

It would be helpful if it were documented what needed to be done to switch to a C++ program.

From what I've read, renaming main.c to main.cpp is important, but beyond that I'm unsure what additional CMake changes need to be made to get everything (especially the STM32 HAL) to play nicely together.

For example, after making this change I seem to get linker issues building for the STM32F3DISCOVERY:

...
CMakeFiles/Project.elf.dir/startup/startup_stm32f303xc.s.obj: In function `LoopFillZerobss':
Project/startup/startup_stm32f303xc.s:111: undefined reference to `main'

[elmot]

@johnthagen thanks for the idea Here is a small but working example Documentation is not written yet. Note the major part of the project is in C, and some classes are used to control LEDs.

[johnthagen]

@elmot Thanks! What do you think of #118? It seems to be working for me so far.

Key to that was adding CXX to:

PROJECT(${projectName} C ASM)