Add contribution guide to repo / RTD

[jsiirola]

This came up at the Dev call today, but we should consider putting together a standard Contribution Guide and include it in the repo and RTD. It should include:

• Code formatting standards (black)
• Expected testing (coverage, duration, etc.)
• Modeling conventions (e.g., referencing https://idaes-pse.readthedocs.io/en/stable/explanations/conventions.html)
• Spelling conventions (e.g., American vs British English)
• Python version support
• PR review process & expectations

[andrewlee94]

I vote for 'Stryne over British or American English.