add http2 section

guide/src/SUMMARY.md

@@ -12,3 +12,4 @@
 - [User sessions](./qs_10.md)
 - [Logging](./qs_11.md)
 - [Static file handling](./qs_12.md)
+- [HTTP/2](./qs_13.md)

guide/src/qs_13.md

@@ -0,0 +1,41 @@
+# HTTP/2
+
+Actix web automatically upgrades connection to `http/2` if possible.
+
+## Negotiation
+
+`HTTP/2` protocol over tls without prior knowlage requires
+[tls alpn](https://tools.ietf.org/html/rfc7301). At the moment only
+`rust-openssl` has support. Turn on `alpn` feature to enable `alpn` negotiation.
+With enable `alpn` feature `HttpServer` provides
+[serve_tls](../actix_web/struct.HttpServer.html#method.serve_tls) method.
+
+```toml
+[dependencies]
+actix-web = { git = "https://github.com/actix/actix-web", features=["alpn"] }
+```
+
+```rust,ignore
+use std::fs::File;
+use actix_web::*;
+
+fn main() {
+    let mut file = File::open("identity.pfx").unwrap();
+    let mut pkcs12 = vec![];
+    file.read_to_end(&mut pkcs12).unwrap();
+    let pkcs12 = Pkcs12::from_der(&pkcs12).unwrap().parse("12345").unwrap();
+
+    HttpServer::new(
+        Application::default("/")
+            .handler("/index.html", index)
+        .serve_tls::<_, ()>("127.0.0.1:8080", pkcs12).unwrap();
+}
+```
+
+Upgrade to `http/2` schema described in
+[rfc section 3.2](https://http2.github.io/http2-spec/#rfc.section.3.2) is not supported.
+Starting `http/2` with prior knowledge is supported for both clear text connection
+and tls connection. [rfc section 3.4](https://http2.github.io/http2-spec/#rfc.section.3.4)
+
+Please check [example](https://github.com/actix/actix-web/tree/master/examples/tls) 
+for concrete example.