The user of-course can already implicitly work out this information from comparing the fee-rate with the actual fee.

I think that it would be useful for people not wanting to do this arithmetic in their head.

const bool fAllowWitness = !(nVersion & SERIALIZE_TRANSACTION_NO_WITNESS);
    READWRITE(*const_cast<int32_t*>(&tx.nVersion));
    unsigned char flags = 0;
    if (ser_action.ForRead()) {
        const_cast<std::vector<CTxIn>*>(&tx.vin)->clear();
        const_cast<std::vector<CTxOut>*>(&tx.vout)->clear();
        const_cast<CTxWitness*>(&tx.wit)->SetNull();
        
        READWRITE(*const_cast<std::vector<CTxIn>*>(&tx.vin));
        if (tx.vin.size() == 0 && fAllowWitness) {
            
            READWRITE(flags);
            if (flags != 0) {
                READWRITE(*const_cast<std::vector<CTxIn>*>(&tx.vin));
                READWRITE(*const_cast<std::vector<CTxOut>*>(&tx.vout));
            }
        } else {
            
            READWRITE(*const_cast<std::vector<CTxOut>*>(&tx.vout));
        }
        if ((flags & 1) && fAllowWitness) {
            
            flags ^= 1;
            const_cast<CTxWitness*>(&tx.wit)->vtxinwit.resize(tx.vin.size());
            READWRITE(tx.wit);
        }
        if (flags) {