Add FreeBird.jl to Other Packages section (#18)

Added FreeBird.jl package information to the index.

Author: yangmr04

index.md

@@ -65,6 +65,8 @@ We also feature a few packages for
 Some other packages for molecular and materials modelling worth mentioning:
 - [Wannier.jl](https://github.com/qiaojunfeng/Wannier.jl):
   Wannier localisation of electronic structures
+- [FreeBird.jl](https://github.com/wexlergroup/FreeBird.jl):
+  Free energy calculators by Bayesian-inspired nested sampling and other integration techniques.
 
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