Mention key size issue in readme

packages/keyv-mysql/README.md

@@ -34,7 +34,7 @@ const keyv = new Keyv('mysql://user:pass@localhost:3306/dbname', {
 });
 ```
 
-
+**Note:** Some MySQL/MariaDB installations won't allow a key size longer than 767 bytes. If you get an error on table creation try reducing `keySize` to 191 or lower. [#5](https://github.com/lukechilds/keyv-sql/issues/5)
 
 ## License