diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1786c5d..c5337c4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,12 +1,14 @@
 CHANGELOG
 =========
 
-0.5.0 (2013-10-XX)
+0.5.0 (2013-10-21)
 ------------------
 
 * Renamed $languages constructor parameters to $locales for both the Client
   and Reader classes.
 * Documentation and code clean-up (Ben Morel).
+* Added the interface `GeoIp2\ProviderInterface`, which is implemented by both
+  `\GeoIp2\Database\Reader` and `\GeoIp2\WebService\Client`.
 
 0.4.0 (2013-07-16)
 ------------------
diff --git a/README.md b/README.md
index e4a9383..1b7967c 100644
--- a/README.md
+++ b/README.md
@@ -52,6 +52,16 @@ You can autoload all dependencies by adding this to your code:
 ```
 require 'vendor/autoload.php';
 ```
+
+### Optional C Extension ###
+
+The [MaxMind DB API](https://github.com/maxmind/MaxMind-DB-Reader-php)
+includes an optional C extension that you may install to dramatically increase
+the performance of lookups in GeoIP2 or GeoLite2 databases. To install, please
+follow the instructions included with that API.
+
+The extension has no effect on web-service lookups.
+
 ## Database Reader ##
 
 ### Usage ###