[docs] Add in notice about requirement for serialization/deserialization for middleware

[womensrights]

Within the using ibc-go --> middleware --> Create a custom IBC middleware section, add in a notice that middleware developers must use the same serialization and deserialization method as in ibc-go's codec: transfertypes.ModuleCdc.[Must]MarshalJSON

For middleware builders this means:

import transfertypes "github.com/cosmos/ibc-go/v10/modules/apps/transfer/types"
transfertypes.ModuleCdc.[Must]MarshalJSON
func MarshalAsIBCDoes(ack channeltypes.Acknowledgement) ([]byte, error) {
	return transfertypes.ModuleCdc.MarshalJSON(&ack)
}

[hungdinh82]

can I work on this issue? @womensrights