Update the Compatibility section of the README

Author: sodabrew

README.md

@@ -490,14 +490,15 @@ This gem is tested with the following Ruby versions on Linux and Mac OS X:
 
 This gem is tested with the following MySQL and MariaDB versions:
 
- * MySQL 5.5, 5.7
+ * MySQL 5.5, 5.6, 5.7
  * MySQL Connector/C 6.0 and 6.1 (primarily on Windows)
- * MariaDB 5.5, 10.0
+ * MariaDB 5.5, 10.0, 10.1
 
-### Active Record
+### Rails / Active Record
 
- * mysql2 0.2.x includes an Active Record driver compatible with AR 2.3 and 3.0
- * mysql2 0.3.x does not include an AR driver because it is included in AR 3.1 and above
+ * mysql2 0.4.x works with Active Record 4.2.5 and higher.
+ * mysql2 0.3.x works with Active Record 3.1 and higher (the AR adapter is now included in AR proper).
+ * mysql2 0.2.x includes an Active Record adapter compatible with AR 2.3 and 3.0, and should not be used with AR 3.1 or higher.
 
 ### Asynchronous Active Record