Add SIAM DS to README

Author: blegat

README.md

@@ -35,6 +35,7 @@ Some presentations on, or using, SumOfSquares (see [blegat/SumOfSquaresSlides](h
 for the source code of the presentations):
 
   * Benoît Legat at [JuMP-dev 2023](https://pretalx.com/juliacon2023/talk/XLT8H3/) [[Slides](https://drive.google.com/file/d/1H-_Ot7tP2g7t95r0K_mTaXvGO1ChUPci/view?usp=drive_link)]
+  * Benoît Legat at [SIAM DS 2023](https://www.siam.org/conferences/cm/conference/ds23) [[Slides](https://drive.google.com/file/d/18isl8aCr_mAsLg9v3eqe-t1BxEP4_TPE/view?usp=drive_link)]
   * Benoît Legat, Marek Kaluba and Tillmann Weisser at INFORMS 2022 [[Slides](https://drive.google.com/file/d/1rlsIxgcnKWT436k4MNenjHfgH0UYRLAB/view?usp=share_link)]
   * Benoît Legat at [POEMA Learning Week 2](http://poema-network.eu/index.php/news-and-events/project-workshops/13-poema-learning-week-2)
   * Benoît Legat, Marek Kaluba and Tillmann Weisser at [JuMP-dev 2021](https://pretalx.com/juliacon2021/talk/L8DTE3/) [[Slides](https://drive.google.com/file/d/1HtArDFNMQ6IYUqRjSWR3JviJp9xLtSlB/view?usp=sharing)]