Add information about using "extern "C"" to the readme

[bricktsre]

I am using a FakeObj.h file to DECLARE the fakes and a FakeObj.c file to DEFINE the fakes. Then I can include the fakes in multiple different files each representing a different test suite. A further complication is my testing code uses gtest and therefore C++ while fff uses C. I ran into some linker issues about not being able to find the _reset functions of the fakes. This was caused by C/C++ name mangling of the functions. The solution to this was to add a extern "C" wrapper to the header file like so

#ifdef __cplusplus
extern "C" {
#endif

DECLARE_FAKE_VALUE_FUNC(func_a);
DECLARE_FAKE_VOID_FUNC(func_b);

// List of fakes used by these unit tests
#define FFF_FAKES_LIST(FUNC)    \
    FUNC(func_a)                            \
    FUNC(func_b) 


#ifdef __cplusplus
}
#endif

I would advocate adding this information to the readme in the "How do I reuse a fake across multiple test-suites?" section.