Update CONTRIBUTING.md

[yardasol]

The CONTRIBUTING.md document has not been changed since 2017. Since then, the project has grown and many more lines of code have been added. The CONTRIBUTING.md document should be updated to ensure high quality contributions. This issue depends on the following:

• A contributor Code of Conduct (Issue Link to the ARFC Code of Conduct in the CONTRIBUTING.md document #129)
• A Contributing Guide (Issue Create templates for issues and pull requests #130)
• A Style Guide (Issue Link to MOOSE code standards in CONTRIBUTING.md #131)

These dependent issues are inspired by the structure of the OpenMC CONTRIBUTING.md document. This list is not comprehensive and additions can be made as needed. In any case, the document should be easy to read and have a clean, clear structure.

This issue can be closed once the new CONTRIBUTING.md document exists.

[yardasol]

closing b/c duplicative of #129, #130, #131. See comments in those issues for more details