Explanation of default features change.

Author: mspiegel

url/src/lib.rs

@@ -73,6 +73,14 @@ assert!(data_url.fragment() == Some(""));
 # run().unwrap();
 ```
 
+## Default Features
+
+Versions `< 3` of the crate have no default features. Versions `>= 3` have the default feature 'std'.
+If you are upgrading across this boundary and you have specified `default-features = false`, then
+you will need to add the 'std' feature or the 'alloc' feature to your dependency.
+The 'std' feature has the same behavior as the previous versions. The 'alloc' feature
+provides no_std support.
+
 ## Serde
 
 Enable the `serde` feature to include `Deserialize` and `Serialize` implementations for `url::Url`.