small documentation update

CONFIG.md

@@ -119,6 +119,12 @@ Adds support for DolphinDOS fast protocol to the IEC bus, using UserPort cable.
 
 Needs some space in KERNAL segment. If unsure - enable.
 
+### `CONFIG_IEC_DOLPHINDOS_FAST`
+
+Faster `LOAD` loop implementation for DolphinDOS protocol.
+
+Needs 20 more bytes in KERNAL segment.
+
 ### `CONFIG_IEC_JIFFYDOS`
 
 Adds support for JiffyDOS fast protocol to the IEC bus.