Update README.md adding 21.3 branch

README.md

@@ -17,6 +17,7 @@ Branch                                                            | Purpose
 [21.0-mc16a](https://gitlab.cern.ch/atlas/athena/tree/21.0-mc16a) | MC16a bugfixes               | Athena,21.0.20.Y
 [21.1](https://gitlab.cern.ch/atlas/athena/tree/21.1)             | HLT and P1 monitoring        | AthenaP1,21.1.X
 [21.2](https://gitlab.cern.ch/atlas/athena/tree/21.2)             | Derivations and Analysis     | AthDerivations, (Ath)AnalysisBase, 21.2.X
+[21.3](https://gitlab.cern.ch/atlas/athena/tree/21.3)             | MC18 Simulation              | Athena, AthSimulation, 21.3.X
 [master](https://gitlab.cern.ch/atlas/athena/tree/master)         | AthenaMT development         | Athena,22.0.X
 
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