Added petgraph code snippet

src/pages/petgraph.md

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+# petgraph
+## graph data structure library
+[![petgraph-badge]][petgraph]
+
+### Example: Drawing the Petersen Graph
+`petgraph` has powerful features for graph representation and algorithms. 
+In the example below, we create and draw a graph in dot format.
+
+*Key concepts:*
+* Creating graphs from vertices and edges
+* Printing in human readable form
+
+```rust
+extern crate petgraph;
+
+use petgraph::Graph;
+use petgraph::dot::{Dot, Config};
+
+fn main() {
+    // Create a new undirected graph, g
+    let mut g = Graph::<u32, u32, petgraph::Undirected>::new_undirected();
+    let w = 0;
+    let mut v = Vec::new();
+
+    // Add 10 vertices to G
+    for i in 1..11 {
+        v.push(g.add_node(i));
+    }
+    
+    // Connect with 15 edges
+    for i in 0..4 {
+        g.add_edge(v[i], v[i + 1], w);
+        g.add_edge(v[i], v[i + 5], w);
+    }
+    g.add_edge(v[0], v[4], w);
+    g.add_edge(v[4], v[9], w);
+
+    g.add_edge(v[5], v[7], w);
+    g.add_edge(v[5], v[8], w);
+    g.add_edge(v[6], v[8], w);
+    g.add_edge(v[6], v[9], w);
+    g.add_edge(v[7], v[9], w);
+    
+    // Print in graphviz dot format
+    println!("{:?}", Dot::with_config(&g, &[Config::EdgeNoLabel]));
+}
+```
+![Petersen Graph](assets/graph.png)
+
+
+<!-- Links -->
+
+[petgraph-badge]: https://img.shields.io/crates/v/petgraph.svg?label=petgraph
+[petgraph]: https://docs.rs/petgraph/0.4.3/petgraph/