DOC: Mention serde feature flag
diff --git a/src/graph_impl/serialization.rs b/src/graph_impl/serialization.rs
index 9dfec94..c400693 100644
--- a/src/graph_impl/serialization.rs
+++ b/src/graph_impl/serialization.rs
@@ -233,6 +233,7 @@
     }
 }
 
+/// Requires crate feature `"serde-1"`
 impl<N, E, Ty, Ix> Serialize for Graph<N, E, Ty, Ix>
     where Ty: EdgeType,
           Ix: IndexType + Serialize,
@@ -291,6 +292,7 @@
 }
 
 
+/// Requires crate feature `"serde-1"`
 impl<'de, N, E, Ty, Ix> Deserialize<'de> for Graph<N, E, Ty, Ix>
     where Ty: EdgeType,
           Ix: IndexType + Deserialize<'de>,
diff --git a/src/graph_impl/stable_graph/serialization.rs b/src/graph_impl/stable_graph/serialization.rs
index 6cfd540..e44aa21 100644
--- a/src/graph_impl/stable_graph/serialization.rs
+++ b/src/graph_impl/stable_graph/serialization.rs
@@ -149,6 +149,7 @@
     }
 }
 
+/// Requires crate feature `"serde-1"`
 impl<N, E, Ty, Ix> Serialize for StableGraph<N, E, Ty, Ix>
     where Ty: EdgeType,
           Ix: IndexType + Serialize,
@@ -214,6 +215,7 @@
     }
 }
 
+/// Requires crate feature `"serde-1"`
 impl<'de, N, E, Ty, Ix> Deserialize<'de> for StableGraph<N, E, Ty, Ix>
     where Ty: EdgeType,
           Ix: IndexType + Deserialize<'de>,
diff --git a/src/lib.rs b/src/lib.rs
index 5791b50..590eaab 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -11,6 +11,8 @@
 //! which is backed by a hash table and the node identifiers are the keys
 //! into the table.
 //!
+//! Optional crate feature flags: `"serde-1"`, see the Readme for more information.
+//!
 #![doc(html_root_url = "https://docs.rs/petgraph/0.4/")]
 
 extern crate fixedbitset;