Adding contribution guide.

# Contributing

## Governance

Contributions to PyGriffin must be made through a merge request (MR) at

https://git-out.gss.anl.gov/nstauff/pygriffin. Even long-term members and

developers must submit changes via MRs to provide a consistent record of the

purpose of contributions as well as to allow other maintainers/developers to

provide feedback on code changes and design decisions.

No merge request should be merged without independent review.

If objections to certain merge requests are raised, the author of the merge

request and reviewers should seek to arrive at a consensus before the MR is

either merged or closed (rejected).

## Code Review Criteria

In order to be considered suitable for inclusion in the PyGriffin repository, the

following criteria must be satisfied for all proposed changes:

  - Changes have a clear purpose and are useful.

  - All continuous integration tests pass in gitlab's CI.

  - If appropriate, test cases are added to regression or unit test suites.

  - Conforms to the [PEP8](https://www.python.org/dev/peps/pep-0008/) style guide where possible.

  - New features/input are documented.

  - No unnecessary external software dependencies are introduced.

## Workflow

We currently encourage a forking workflow for contributions to PyGrifin. This

workflow involves the following steps:

  1. Fork the main PyGriffin repository under your user account at

     https://git-oug.gss.anl.gov. You can reach out to nstauff@anl.gov about

     creating an account.

  2. Clone your fork of PyGriffin locally to make changes to the code.

  ```shell

  $ git clone https://git-out.gss.anl.gov/your_username/pygriffin

  $ cd pygriffin

  $ git checkout -b new_branch main

  ```

  3. Make your changes in your local branch and push the branch to your fork

     when ready.

  4. Open a merge request on gitlab to the main PyGriffin repository with a

     description of the changes along with motivation for these changes if they

     aren't adressing an open issue in PyGriffin.

  5. Another maintainer/developer will review your merge request based on the

     criteria above. Any issues with the merge request can be discussed directly

     on in the MR.

  6. When the merge request has been approved, it will be merged into the main

     branch of PyGriffin.

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