DOC: update documentation.

Add documentation covering:
  - developer information about architecture
  - project structure and building
  - project overview and service descriptions
  - the portability model for supporting hardware

Signed-off-by: Julian Hall <julian.hall@arm.com>
Co-Authored-By: Gyorgy Szing <gyorgy.szing@gmail.com>
Change-Id: I8bf9c01a66350719d82a7ca2bc1c78a8ab17978d
diff --git a/docs/project/todo.rst b/docs/project/todo.rst
new file mode 100644
index 0000000..1739eab
--- /dev/null
+++ b/docs/project/todo.rst
@@ -0,0 +1,22 @@
+TODO list
+=========
+
+This file collects information on planned future changes and issues to be
+addressed. For managing the TODO list below, the `Sphinx TODO extension`_ is
+used.
+
+.. todo:: Find a solution to make the rendering of TODO entries less intrusive
+          at their original location.
+
+List of TODO entries in the project
+-----------------------------------
+
+.. todolist::
+
+--------------
+
+.. _`Sphinx TODO extension`: https://www.sphinx-doc.org/en/master/usage/extensions/todo.html
+
+*Copyright (c) 2020-2021, Arm Limited and Contributors. All rights reserved.*
+
+SPDX-License-Identifier: BSD-3-Clause
\ No newline at end of file