prep change log for v0.13 release

CHANGELOG.md

@@ -2,6 +2,9 @@ PowerModels.jl Change Log
 =========================
 
 ### Staged
+- nothing
+
+### v0.13.0
 - Added SOC formulation of the storage model
 - Simplified PowerModel types (#192) (breaking)
 - Improved multi-conductor support in the storage model (#572) (breaking)
@@ -12,9 +15,6 @@ PowerModels.jl Change Log
 - Made baseMVA not a global key in multinetwork models (breaking)
 - Fixed bug in bus_storage_lookup (#598)
 
-### Staged
-- nothing
-
 ### v0.12.5
 - Update to InfrastructureModels v0.3 and JuMP v0.20
 - Fixed power balance duals in w-space formulations (#591)