Doc: convert markdown files to rst

The official documentation format for TF-M is becoming Restructured
Text. This change converts the existing .md files to the new format.

Due to support for documentation of external code (/lib) md support is
kept.

Change-Id: I3c1aef5d22442e0c7c362a40a8d344a266dc0940
Signed-off-by: Gyorgy Szing <Gyorgy.Szing@arm.com>
diff --git a/contributing.rst b/contributing.rst
new file mode 100644
index 0000000..691dcde
--- /dev/null
+++ b/contributing.rst
@@ -0,0 +1,42 @@
+Contributing to Trusted Firmware M
+==================================
+
+Contributions to TF-M project need to follow the process below.
+
+``Note`` Please contact :doc:`maintainers` for any questions.
+
+-  Create an issue in http://issues.trustedfirmware.org
+   to keep others informed about your ongoing work.
+-  If it is a major change then please discuss the design with
+   maintainers.
+-  Clone the TF-M code on your own machine from
+   http://git.trustedfirmware.org/trusted-firmware-m.git
+-  Follow the :doc:`Coding Guide </docs/coding_guide>` for the TF-M
+   project.
+-  Make your changes in logical chunks to help reviewers.
+-  Update relevant documentation.
+-  Test your changes and add details to the commit description.
+-  Add the issue number and details to commit description as well.
+-  The code is accepted under :doc:`DCO </docs/dco>`, Developer
+   Certificate of Origin, so you must add following fields to your commit
+   description.::
+
+       Author: Full Name <email address>
+       Signed-off-by: Full Name <email address>
+      
+       Note: Sign off authority needs to adhere to the [DCO](./dco.txt) rules.
+
+-  You may add other fields in the commit message.
+-  Submit your patch for review at
+   http://review.trustedfirmware.org
+-  Maintainers will be notified and they will start review process.
+-  You may be asked to provide further details or make additional
+   changes.
+-  You can discuss further with maintainer(s) offline if necessary.
+-  | Once the change is approved by maintainers, the patch can be
+     submitted
+   | either by maintainer or patch owner.
+
+--------------
+
+*Copyright (c) 2017-2019, Arm Limited. All rights reserved.*