Add CONTRIBUTING.md guide

The following discussion from !11 (merged) should be addressed:

• @steveazz started a discussion: (+1 comment)

  Should this document be reviewed but a technical writer?

TODO

Add to CONTRIBUTING.MD

## Style Guide

We follow GitLab's [Go standards and style guidelines](https://docs.gitlab.com/ee/development/go_guide/).

## Changelog

GitLab's Release CLI keeps a [changelog](CHANGELOG.md) which is generated when a new release
is created. The changelog is generated from entries that are included on each
merge request. To generate an entry on your branch run:
`_support/changelog "Change descriptions"`.

After the merge request is created, the ID of the merge request needs to be set
in the generated file. If you already know the merge request ID, run:
`_support/changelog -m <ID> "Change descriptions"`.

Any new merge request must contain either a new entry or a
justification in the merge request description why no changelog entry is needed.


## Release CLI Maintainers

| Maintainer         |
|--------------------|
|@steveazz|
|@jaime|

## Development Process

GitLab's Release CLI follows the engineering process as described in the [handbook][eng-process],
with the exception that our [issue tracker][release-cli-issues] is on the GitLab's Release CLI
project and there's no distinction between developers and maintainers. Every team
member is equally responsible for a successful master pipeline and fixing security
issues.

Merge requests need to **approval by at least one
[GitLab Release Management team member](https://about.gitlab.com/handbook/engineering/development/ci-cd/release/release-management/#team-members)
AND a [maintainer](#release-cli-maintainers) **.

[eng-process]: https://about.gitlab.com/handbook/engineering/workflow/
[release-cli-issues]: https://gitlab.com/gitlab-org/release-cli/issues/