add specialization to docs

README.md

@@ -75,7 +75,7 @@ features = ["extension-module"]
 **`src/lib.rs`**
 
 ```rust
-#![feature(proc_macro)]
+#![feature(proc_macro, specialization)]
 
 extern crate pyo3;
 use pyo3::{py, PyResult, Python, PyModule};

guide/src/module.md

@@ -4,6 +4,7 @@ Python module generation is powered by unstable [Procedural Macros](https://doc.
 
 ```rust
 #![feature(proc_macro)]
+
 extern crate pyo3;
 # fn main() {}
 ```

guide/src/overview.md

@@ -70,7 +70,7 @@ features = ["extension-module"]
 **`src/lib.rs`**
 
 ```rust
-#![feature(proc_macro)]
+#![feature(proc_macro, specialization)]
 
 extern crate pyo3;
 use pyo3::{py, PyResult, Python, PyModule};

src/lib.rs

@@ -78,7 +78,7 @@
 //! # Example
 //!
 //! ```rust
-//! #![feature(proc_macro)]
+//! #![feature(proc_macro, specialization)]
 //!
 //! extern crate pyo3;
 //! use pyo3::{py, Python, PyResult, PyObject, PyModule, PyString};