Add documentation about using MPITrampoline

[giordano]

I was looking into MPITrampoline for solving #835, but I realised we have no documentation at all about how to use MPITrampoline to forward MPI calls to a system library. For example, users supposed to compile the wrapper themselves? In any case, writing down the process would be good. @eschnett can I assign this to you?.

[eschnett]

Yes, will do that.