Add example for Graph with illustration

Author: bluss

Makefile

@@ -7,7 +7,7 @@ FEATURES =
 
 VERSIONS = $(patsubst %,target/VERS/%,$(DOCCRATES))
 
-docs: mkdocs subst $(RMDOCS)
+docs: mkdocs mksvgs subst $(RMDOCS)
 
 # https://www.gnu.org/software/make/manual/html_node/Automatic-Variables.html
 $(VERSIONS): Cargo.toml
@@ -30,5 +30,9 @@ $(RMDOCS): mkdocs
 	rm -r ./doc/$@
 	sed -i "/searchIndex\['$@'\]/d" doc/search-index.js
 
+mksvgs: mkdocs graph-example.dot
+	dot -Tsvg < ./graph-example.dot > graph-example.svg
+	mv graph-example.svg ./doc/petgraph/graph/
 
-.PHONY: docs mkdocs subst $(DOCCRATES) $(RMDOCS)
+
+.PHONY: docs mkdocs mksvgs subst $(DOCCRATES) $(RMDOCS)

graph-example.dot

@@ -0,0 +1,15 @@
+digraph {
+rankdir = "LR";
+splines = true;
+
+    0 [label="petgraph"]
+    1 [label="fixedbitset"]
+    2 [label="quickcheck"]
+    3 [label="rand"]
+    4 [label="libc"]
+    0 -> 1
+    0 -> 2
+    2 -> 3
+    3 -> 4
+    2 -> 4
+}

src/graph.rs

@@ -205,6 +205,23 @@ impl<E, Ix: IndexType = DefIndex> Edge<E, Ix>
 ///
 /// Based on the graph datastructure used in rustc.
 ///
+/// ```
+/// use petgraph::Graph;
+/// 
+/// let mut deps = Graph::<&str, &str>::new();
+/// let pg = deps.add_node("petgraph");
+/// let fb = deps.add_node("fixedbitset");
+/// let qc = deps.add_node("quickcheck");
+/// let rand = deps.add_node("rand");
+/// let libc = deps.add_node("libc");
+/// deps.extend_with_edges(&[
+///     (pg, fb), (pg, qc),
+///     (qc, rand), (rand, libc), (qc, libc),
+/// ]);
+/// ```
+///
+/// ![graph-example](graph-example.svg)
+///
 /// ### Graph Indices
 ///
 /// The graph maintains unique indices for nodes and edges, and node and edge