add docs

docs/FEATURE_SET.md

+<table style="border-collapse: collapse; border: none;">
+  <tr style="border-collapse: collapse; border: none;">
+    <td style="border-collapse: collapse; border: none;">
+      <a href="http://www.openairinterface.org/">
+         <img src="./images/oai_final_logo.png" alt="" border=3 height=50 width=150>
+         </img>
+      </a>
+    </td>
+    <td style="border-collapse: collapse; border: none; vertical-align: center;">
+      <b><font size = "5">OpenAirInterface SMF Feature Set</font></b>
+    </td>
+  </tr>
+</table>
+
+**Table of Contents**
+
+1. [5GC Service Based Architecture](#1-5gc-service-based-architecture)
+2. [OAI SMF Available Interfaces](#2-oai-smf-available-interfaces)
+3. [OAI SMF Feature List](#3-oai-smf-feature-list)
+
+# 1. 5GC Service Based Architecture #
+
+![5GC SBA](./images/5gc_sba.png)
+
+#2. OAI SMF Available Interfaces #
+
+| **ID** | **Interface** | **Status**         | **Comment**                                          |
+| ------ | ------------- | ------------------ | -----------------------------------------------------|
+| 1      | N4            | :heavy_check_mark: | between SMF and UPF (PFCP)                           |
+| 2      | N7            | :x:                | between SMF and PCF                                  |
+| 3      | N10           | :heavy_check_mark: | between SMF and UDM (Nudm_SubscriberDataManagement)  |
+| 4      | N11           | :heavy_check_mark: | between SMF and AMF (Nsmf_PDU_Session Services,      |
+|        |               |                    |     Namf_N1N2MessageTransfer)                        |
+
+
+#3. OAI SMF Feature List #
+
+Based on document **3GPP TS 23.501 v16.0.0 §6.2.2**.
+
+| **ID** | **Classification**                                                  | **Status**         | **Comments**                                |
+| ------ | ------------------------------------------------------------------- | ------------------ | ------------------------------------------- |
+| 1      | Session Management (Session Establishment/Modification/Release)     | :heavy_check_mark: |                                             |
+| 2      | UE IP address allocation & management​                               | :heavy_check_mark: | Only support static ipv4 address allocation |
+| 3      | DHCPv4 (server and client) and DHCPv6 (server and client) function  | :x:                |                                             |
+| 4      | Respond to ARP requests and/or IPv6 Neighbour Solicitation requests | :x:                | Local configuration in SMF                  |
+| 5      | Selection of UPF function​                                           | :x:                |                                             |
+| 6      | Configures traffic steering at UPF                                  | :x:                |                                             |
+| 7      | Termination of interfaces towards PCFs                              | :x:                |                                             |
+| 8      | Lawful intercept                                                    | :x:                |                                             |
+| 8      | Charging data collection and support of charging interfaces         | :x:                |                                             |
+| 10     | Termination of SM parts of NAS messages                             | :heavy_check_mark: |                                             |
+| 11     | Downlink Data Notification                                          | :heavy_check_mark: |                                             |
+| 12     | Determine SSC mode of a session​                                     | :heavy_check_mark: | Only support SSC mode 1​                     |
+| 13     | Initiator of AN specific SM information, sent via AMF over N2 to AN | :heavy_check_mark: |                                             |
+| 14     | Support for Control Plane CIoT 5GS Optimisation                     | :x:                |                                             |
+| 15     | Support of header compression. ​                                     | :x:                |                                             |
+| 16     | Act as I-SMF in deployments                                         | :x:                |                                             |
+| 17     | Provisioning of external parameters                                 | :x:                |                                             |
+
+