at_c: add _init and _free convention 8 to CONTRIBUTING.md

[JeremyTubongbanua]

Using this as a note to self to do this

We follow MbedTLS' OOP approach in C.

Our modified convention of this approach will state that:

1. for every _init function, a _free function should be called at the end of the struct's lifetime for future proofing and clean memory management.
2. _init and _free functions should of type void (this raises the question of "what happens if an error happens in the function" and this is answered in the next rule)
3. _init and _free function bodies should be immune to run-time errors and any error that is of no fault of the caller. Preconditions should be defined in the function docs and if the caller of the function does not follow those preconditions, then it is of no fault of the function itself, and it is not the function's responsibility to return an error code, hence why we allow ourselves to set the function type to void. This also means that malloc cannot be used in _init because malloc is prone to memory full run-time error, for example.