Add note on HPIPM evaluation to the readme

README.md

@@ -75,6 +75,7 @@ Here are some known areas of improvement for this benchmark:
 - [Cold start only:](https://github.com/qpsolvers/qpbenchmark/issues/101) we don't evaluate warm-start performance for now.
 - [CPU thermal throttling:](https://github.com/qpsolvers/qpbenchmark/issues/88) the benchmark currently does not check the status of CPU thermal throttling.
     - Adding this feature is a good way to [start contributing](https://github.com/qpsolvers/qpbenchmark/labels/good%20first%20issue) to the benchmark.
+- [HPIPM evaluation:](https://github.com/qpsolvers/qpbenchmark/issues/122) currently we evaluate HPIPM against a duality-gap tolerance but the solver is asked to solve a complementarity-slackness condition.
 
 Check out the issue tracker for ongoing works and future improvements.