Update FAPI style interface for NB-IoT documents.

targets/DOCS/NB-IoT_Docs/Readme

@@ -13,6 +13,8 @@ NB-IoT_Docs Folder
             
             5. NB_MAC_SAP - This document is mainly describling about the Service Access Point (primitives) between the MAC and other layers in OAI.
             
+            6. NB_MAC_Fapi_interface - This document is mainly for the FAPI style interface between OAI MAC and PHY for NB-IoT, and described how old messages in OAI go through in the FAPI style interface.
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