docs: Update coding standard blurb in contributions readme

docs: Update coding standard blurb in contributions readme

Observed issue
==============

The CodingStyle document now includes style guidelines for Python,
Shell (bash), and legacy C code.

Solution
========

* Fix wording in the first sentence and use a relative link to the file
* Remove paragraph that other languages do not have a coding style
* Add a link to the test README

Known drawbacks
===============

None.

Change-Id: I057ae8d51afaa07892aa40b47c2f1716574b3b06
Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>