GitLab

GitHub notices a CONTRIBUTING file in a repo's root and will
automatically
add a link to this file on the page when a contributor creates an Issue
or opens a Pull Request. (Expectation is CONTRIBUTING will have
information about how to contribute to the project, format code,
test fixes, and submit patches.

We also want to have this document accessible from our technical docs,
and not duplicate the content, so add linkage to make this work.

The zephyrproject github wiki article that contributed to this new
CONTRIBUTING doc will be made into a reference to this new doc once
this PR is approved and merged.

Replaces PR #929

Jira: ZEP-2085
Signed-off-by: David B. Kinder <david.b.kinder@intel.com>