trades: add note on Kelly bet for biden term trade

src/content/trades/biden-finishes-term.mdx

@@ -31,9 +31,12 @@ I purchased 20 shares at an average price of 91.6¢, for a total of $18.28 befor
 I think my entry was fine although I probably could have stood to
 spend some more time and effort on my bet sizing.
 This was an extremely good price which I mostly didn't take advantage of,
-and in the future I should at least spend some time estimating what the [Kelly bet](https://en.wikipedia.org/wiki/Kelly_criterion) would be.
+and in the future I should at least spend some time estimating what the [Kelly bet](https://en.wikipedia.org/wiki/Kelly_criterion) would be.[^kelly_bet]
 On the other hand, I don't want to go overboard and end up essentially just gambling.
 
+[^kelly_bet]: In this case, the 99% forecast earlier results in a Kelly bet of just under 89% of the available bankroll.
+I would of course not be this confident given my surprise at the price in the first place.
+
 My exit was similarly not great.
 I set a sell limit order for half my shares at 98¢,
 but on reflection I realize that there was no principled reason for this: