Document that Mio report OOB data in Event::is_readable

Reporting Out-of-band (OOB) as readable it could leave applications open
to DoS attacks. However because Mio uses edge-triggers most applications
won't actually be effected.

src/event/event.rs

@@ -25,6 +25,15 @@ impl Event {
     }
 
     /// Returns true if the event contains readable readiness.
+    ///
+    /// # Notes
+    ///
+    /// Out-of-band (OOB) data also triggers readable events. But must
+    /// application don't actually read OOB data, this could leave an
+    /// application open to a Denial-of-Service (Dos) attack, see
+    /// <https://github.com/sandstorm-io/sandstorm-website/blob/58f93346028c0576e8147627667328eaaf4be9fa/_posts/2015-04-08-osx-security-bug.md>.
+    /// However because Mio uses edge-triggers it will not result in an infinite
+    /// loop as described in the article above.
     pub fn is_readable(&self) -> bool {
         sys::event::is_readable(&self.inner)
     }