TLDR, not recommended.
ReadyBoost (and any other kind of disk caching solution) works by creating an intermediate buffer between the fast RAM and slow HDD. The caveat with HDDs is that they are pretty okay in terms of sequential operations, but do terribly in random operations. And it is random reads that determines your overall system responsiveness. ReadyBoost intends on improving just this, so you want to make your intermediate storage medium have as much random read performance as possible.
Random reads and writes are measured in iops. HDDs usually fall between 300-2000; SSDs can typically do 50K-100K. This is why SSDs are so much more responsive. How many iops you can get out of your SD card is not only dependent on the quality of the card, but also that of the card reader. Sadly, if your card reader is USB based, I would not expect any more than 5K. So I would say the performance improvement you get would be very limited.
Another thing to note is that USB and SD interfaces are really not designed to be permanently connected. You can very easily accidentally unplug them, in which case I really don't think ReadyBoost will like that.
Alternatively, I would simply recommend to go buy a larger capacity RAM module and swap out the one you currently have, assuming RAM is not soldered on your laptop. That would be much more effective and reliable while only needing slightly more effort.