Add general contribution guide

[hajdik]

Purpose

We have a contribution guide for pyOptSparse but most of the info is generally applicable to the lab's codes so it can be centralized. After this is made, other repos can link to it from their docs and include additional info specific to the individual repos on their own contribution page.

Expected time until merged

Soon, not a major change

Type of change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (non-backwards-compatible fix or feature)
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Documentation update
• Maintenance update
• Other (please describe)

Testing

N/A

Checklist

• I have run flake8 and black to make sure the Python code adheres to PEP-8 and is consistently formatted
• I have formatted the Fortran code with fprettify or C/C++ code with clang-format as applicable
• I have run unit and regression tests which pass locally with my changes
• I have added new tests that prove my fix is effective or that my feature works
• I have added necessary documentation

[sseraj]

A few more comments

[sseraj]

Looks good!