Add pubsubhubbub feed information to template

Author: samueltardieu

templates/releases_feed.hbs

@@ -4,6 +4,8 @@
 <subtitle>Recent Rust crates</subtitle>
 <link href="https://docs.rs/releases/feed" rel="self" />
 <link href="https://docs.rs/" />
+<link href="https://pubsubhubbub.appspot.com" rel="hub" />
+<link href="https://pubsubhubbub.superfeedr.com" rel="hub" />
 <id>urn:docs-rs:{{cratesfyi_version_safe}}</id>
 <updated>{{content[0].release_time_rfc3339}}</updated>
 {{#each content}}