Added empty simple graphs construction in construction.md (#399)

Author: g-adel

docs/src/first_steps/construction.md

@@ -6,7 +6,7 @@ _Graphs.jl_ provides a number of methods for creating a graph. These include too
 
 ### Standard generators
 
-_Graphs.jl_ implements numerous graph generators, including random graph generators, constructors for classic graphs, numerous small graphs with familiar topologies, and random and static graphs embedded in Euclidean space.
+_Graphs.jl_ implements numerous graph generators, including random graph generators, constructors for classic graphs, numerous small graphs with familiar topologies, and random and static graphs embedded in Euclidean space. An empty simple Graph can be constructed using `g = SimpleGraph()` and similary `g = SimpleDiGraph()` for directed graphs.
 See [Generators for common graphs](@ref) for a complete list of available templates.
 
 ### Datasets