please add some points to the ReadMe limitations section

[mattloulou]

Request to add more information in limitations:

From my experimentation, it seems that your program only supports C functions that have void return types,
Additionally, it seems that uint64_t from the header <stdint.h> is not supported.
So, for my future reference, I'd appreciate if you add those 3 facts to the "limitations" section of the readme:

- C functions must have void return types
- `uint64_t` from the header <stdint.h> is not supported
- C source file names should not begin with `_`