Added density calculation

Graphs/Density.js

@@ -0,0 +1,11 @@
+/*
+The density of a network is a measure of how many edges exist proportional to
+how many edges would exist in a complete network (where all possible edges).
+https://networkx.org/documentation/networkx-1.9/reference/generated/networkx.classes.function.density.html
+*/
+function density (numberOfNodes, numberOfEdges, isDirected = false) {
+  const multi = isDirected ? 1 : 2
+  return (multi * numberOfEdges) / (numberOfNodes * (numberOfNodes - 1))
+}
+
+(() => { console.log(density(10, 2)) })()