update docs

docs/src/index.md

@@ -31,6 +31,18 @@ julia> metabolites
   ⋮
 
 julia> S
+77×77 SparseArrays.SparseMatrixCSC{Float64,Int64} with 308 stored entries:
+  [60,  1]  =  -1.0
+  [68,  1]  =  1.0
+  [1 ,  2]  =  1.0
+  [6 ,  2]  =  -1.0
+  ⋮
+  [23, 76]  =  1.0
+  [56, 76]  =  -1.0
+  [30, 77]  =  -1.0
+  [48, 77]  =  1.0
+
+julia> Matrix(S)
 77×77 Array{Float64,2}:
  0.0   1.0  0.0  0.0  0.0  0.0  0.0  …  0.0   0.0  0.0   0.0  0.0  0.0  0.0
  0.0   0.0  0.0  0.0  0.0  0.0  0.0     0.0   0.0  0.0   0.0  0.0  0.0  0.0