Document rust-encoding dependency.

README.md

@@ -4,3 +4,5 @@ rust-url
 Rust implementation of the [URL Standard](http://url.spec.whatwg.org/).
 
 See [Rust bug #10707](https://github.com/mozilla/rust/issues/10707).
+
+This depends on [rust-encoding](https://github.com/lifthrasiir/rust-encoding).