Proposal: Create contribution guidelines for InVEST

[phargogh]

Although we haven't had many requests for contributions, the question of "do you accept contributions" does occasionally come up and it's never an easy answer because the process isn't documented. I propose that we correct this situation with a starter contribution guide, fully understanding that we'll update it as we need to.

What I propose is to consolidate what we already practice into a single document:

1. Contributions are welcome!
2. New InVEST models, model deprecations and major changes require PSC review (and link to that process if/when we have it)
3. For possible issues with equations and/or science, make an issue and let's discuss!
4. Process for proposing a change:
   1. If an issue doesn't already exist for the change, create it.
   2. Fork the repository, creating a development branch if you so desire
   3. When complete, PR the changes into main (probably)
5. Coding conventions we follow for python and javascript, test coverage, other things we look for.

This document is not intended to take the place of an updated README (see #670) but rather complement it, so I suspect that things like "how do I install InVEST" and where to get support would belong in the README.

Estimated remaining effort

1-3 days of concerted effort, but I imagine running this by OTL will take some time waiting for a response. There remains at least one outstanding legal question:

• [ ] Do we need some specific indication about granting Stanford an irrevocable license to modify, distribute, etc. or something like that?

[phargogh]

I've got a rough draft of this doc at https://docs.google.com/document/d/1YOaIW6z1foGCCy9uw5oefYfIoOcE1w71KW5r7F0gm-s/edit# (Stanford auth required)