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 NRF Feature Set</font></b>
+    </td>
+  </tr>
+</table>
+
+**Table of Contents**
+
+1. [5GC Service Based Architecture](#1-5gc-service-based-architecture)
+2. [OAI NRF Available Interfaces](#2-oai-nrf-available-interfaces)
+3. [OAI NRF Feature List](#3-oai-nrf-feature-list)
+
+# 1. 5GC Service Based Architecture #
+
+![5GC SBA](./images/5gc_sba.png)
+
+# 2. OAI NRF Available Interfaces #
+
+| **ID** | **Interface** | **Status**         | **Comment**                                                               |
+| ------ | ------------- | ------------------ | -------------------------------------|
+| 1      | Nnrf          | :heavy_check_mark: | Between NRF and other NFs            |
+                                                             
+
+# 3. OAI NRF Feature List #
+
+Based on document **3GPP TS 23.501 v16.0.0 (Section 6.2.6)**.
+
+| **ID** | **Classification**                                                              | **Status**         | **Comments**     |
+| ------ | ------------------------------------------------------------------------------- | ------------------ | ---------------- |
+| 1      | Supports service discovery function                                             | :heavy_check_mark: |                  |
+| 2      | Maintains the NF profile of available NF instances and their supported services | :heavy_check_mark: |                  |
+