Add Markdown formatting documentation

This patch addresses reviewers need to point to a standard to
follow for Markdown formatting when contributing to
the Airship project.

It is expected that when code is submitted, it should have
markdownlint run against it before hand. Later these should be
enforced at the gate.

Change-Id: I7ddf0cf36456dedf1382bf52c61587c0f8e3ef8d

doc/source/develop/code-conventions.rst

@@ -109,6 +109,7 @@ Language         Tools Used
 ===============  ======================================
 Ansible          ansible-lint
 Go               gofmt
+Markdown         markdownlint
 Python           YAPF, Flake8
 ===============  ======================================
 
@@ -126,6 +127,12 @@ Go code should be formatted using gofmt. When using gofmt be sure to use the
 
   gofmt -s /path/to/file.go
 
+Markdown Formatting
+~~~~~~~~~~~~~~~~~~~
+
+Markdown code (documentation) should be linted to be conformant to the
+standards checked by `markdownlint`_ project.
+
 Python PEP-8 Formatting
 ~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -279,3 +286,4 @@ prevail.
 .. _helm-toolkit: https://git.openstack.org/cgit/openstack/openstack-helm-infra/tree/helm-toolkit
 .. _Openstack-Helm: https://wiki.openstack.org/wiki/Openstack-helm
 .. _ansible-lint: https://github.com/ansible/ansible-lint
+.. _markdownlint: https://github.com/DavidAnson/markdownlint