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/developer/writing-documentation.rst b/docs/developer/writing-documentation.rst
new file mode 100644
index 0000000..850d9cd
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+Writing documentation
+=====================
+
+|TS| is documented using `Sphinx`_, which in turn uses Docutils and `Restructured Text`_ (|REST| hereafter).
+
+The source files for the documents are in the *docs* directory of the |TS_REPO|.
+
+The preferred output format is *HTML*, and other formats may or may not work.
+
+
+Section Headings
+----------------
+
+In order to avoid problems if documents include each other, it is important to follow a consistent section heading
+style. Please use at most five heading levels. Please use the following style::
+
+    First-Level Title
+    =================
+
+    Second-Level Title
+    ------------------
+
+    Third-Level Title
+    '''''''''''''''''
+
+    Forth-level Title
+    """""""""""""""""
+
+    Fifth-level Title
+    ~~~~~~~~~~~~~~~~~
+
+
+Inline documentation
+--------------------
+
+To get all information integrated into a single document the project uses Sphinx extensions to allow capturing inline
+documentation into this manual.
+
+
+CMake
+'''''
+
+.. todo:: Add content about how to document cmake scripts.
+
+
+--------------
+
+.. _`Restructured Text`: https://docutils.sourceforge.io/rst.html
+.. _`Sphinx`: https://www.sphinx-doc.org
+
+*Copyright (c) 2020-2021, Arm Limited and Contributors. All rights reserved.*
+
+SPDX-License-Identifier: BSD-3-Clause