Add a link to CMU Distributed Systems course page (alshedivat#193)

Author: akindofyoga

README.md

@@ -67,7 +67,7 @@ Feel free to add your own page(s) by sending a PR.
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 CMU PGM (<a href="https://sailinglab.github.io/pgm-spring-2019/" target="_blank">S-19</a>),
 CMU DeepRL (<a href="https://cmudeeprl.github.io/703website_f19/" target="_blank">F-19</a>, <a href="https://cmudeeprl.github.io/Spring202010403website/" target="_blank">S-20</a>, <a href="https://cmudeeprl.github.io/703website/" target="_blank">F-20</a>),
-CMU MMML (<a href="https://cmu-multicomp-lab.github.io/mmml-course/fall2020/" target="_blank">F-20</a>)
+CMU MMML (<a href="https://cmu-multicomp-lab.github.io/mmml-course/fall2020/" target="_blank">F-20</a>), CMU Distributed Systems (<a href="https://andrew.cmu.edu/course/15-440/" target="_blank">S-21</a>)
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