Add description of add_tl_voltages!

src/Data.jl

@@ -60,6 +60,9 @@ end
 
 #*------------------------------------------------------------------------------
 
+#=
+Determines voltage levels of transmission lines and adds them to the NDD.
+=#
 function add_tl_voltages!(network_data::Dict{String,<:Any})
     voltages = Dict{String,Any}(
         "baseMVA"  => 100.0,