DOC: add architecture

Author: ciusji

README.md

@@ -19,6 +19,8 @@
 An extensible environment for interactive and reproducible computing, based on the
 Jupyter Notebook and Architecture. [Currently ready for users.](https://blog.jupyter.org/jupyterlab-is-ready-for-users-5a6f039b8906)
 
+![architecture](/jupyterlab/staging/assets/architecture.jpg)
+
 [JupyterLab](http://jupyterlab.readthedocs.io/en/stable/) is the next-generation user interface for [Project Jupyter](https://jupyter.org) offering
 all the familiar building blocks of the classic Jupyter Notebook (notebook,
 terminal, text editor, file browser, rich outputs, etc.) in a flexible and