Update README.md (add PEtab reference)

Author: TorkelE

README.md

@@ -120,6 +120,7 @@ Catalyst.jl](https://www.youtube.com/watch?v=5p1PJE5A5Jw).
   resulting stochastic chemical kinetics with delays models.
 - [BondGraphs.jl](https://github.com/jedforrest/BondGraphs.jl) a package for
   constructing and analyzing bond graphs models, which can take Catalyst models as input.
+- [PEtab.jl](https://github.com/sebapersson/PEtab.jl) a package that implements the PEtab format for fitting reaction network ODEs to data. Input can be provided either as SBML files or as Catalyst `ReactionSystem`s.
 
 
 ## Illustrative examples