Update DataFrame.jl

add description

Author: emfeltham

src/DataFrame.jl

@@ -4,6 +4,9 @@ import DataFrames.DataFrame
     DataFrame(gr::T; type = :node) where T <:AbstractMetaGraph
 
 Construct a DataFrame from a MetaGraph from either its node or edge properties.
+`gr` is a MetaGraph.
+`type` is a Symbol valued either :node or :edge such that the DataFrame is populated with node or edge
+properties stored in `gr`.
 """
 function DataFrame(gr::T; type = :node) where T <:AbstractMetaGraph
     fl, prps, en, nu = if type == :node