Add two figures to README.md

README.md

@@ -1,4 +1,5 @@
-# Occlum
+![Occlum logo](docs/images/logo.png)
+## <!-- render a nicely looking grey line to separate the logo from the content  -->
 [![All Contributors](https://img.shields.io/badge/all_contributors-7-orange.svg?style=flat-square)](CONTRIBUTORS.md)
 [![Build Status](https://travis-ci.com/occlum/occlum.svg?branch=master)](https://travis-ci.com/occlum/occlum)
 
@@ -221,6 +222,12 @@ Occlum is being actively developed. We now focus on implementing more system cal
 
 While this project is still not mature or stable (we are halfway through reaching version 1.0.0), we have used Occlum to port many real-world applications (like Tensorflow Lite, XGBoost, GCC, Lighttpd, etc.) to SGX with little or no source code modifications. We believe that the current implementation of Occlum is already useful to many users and ready to be deployed in some use cases.
 
+## How about the Internal Working?
+
+The high-level architecture of Occlum is summarized in the figure below:
+
+![Arch Overview](docs/images/arch_overview.png)
+
 ## Why the Name?
 
 The project name Occlum stems from the word *Occlumency* coined in Harry Potter series by J. K. Rowling. In *Harry Potter and the Order of Phoenix*, Occlumency is described as: