history file update

HISTORY.md

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 # Breaking updates and feature summaries across releases
 
 ## Catalyst unreleased (master branch)
+- Update how compounds are created. E.g. use
+```julia
+@variables t C(t) O(t)
+@compound CO2 ~ C + 2O
+```
+to create a compound species `CO2` that consists of `C` and 2 `O`.
+- Added documentation for chemistry related functionality (compound creation and reaction balancing).
 
 ## Catalyst 13.5
 - Added a CatalystHomotopyContinuationExtension extension, which exports the `hc_steady_state` function if HomotopyContinuation is exported. `hc_steady_state` finds the steady states of a reactin system using the homotopy continuation method. This feature is only available for julia versions 1.9+. Example: