Add documentation on how to use the ITTI simulator

doc/ITTISIM.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">Running L3 ITTI simulator</font></b>
+    </td>
+  </tr>
+</table>
+
+This page is valid on the following branches:
+
+- `develop` starting from tag `2020.w48`
+
+# 1. Building the ITTI simulator.
+
+The ITTI simulator is available directly from the standard build.
+
+```bash
+$ source oaienv
+$ cd cmake_targets
+$ sudo ./build_oai -x -w None -c ?ittiSIM
+```
+
+# 2. Running the ITTI simulator.
+
+The ITTI simulator establishes ITTI-threaded communication between the gNB RRC task and the UE RRC task.
+
+This allows to test the sequence of NGAP/RRC/NAS messages.
+
+The main limitations are:
+
+- NAS is a simple stub that just sends and receives messages
+- only initial Attach sequence
+
+## 2.1. Starting the ITTI simulator
+
+The ITTI simulator is able to run with a connected 5GC or without any.
+The develop branch tag `2020.w48` only works RRC without 5GC connection.
+
+```bash
+$ sudo -E ./ran_build/build/nr-ittisim -O gnb.conf
+```
+