Create a contributing.md for onboarding new developers

[putmantime]

Background:
We would like to lower the barrier for bringing on new monarch ingest developers and support standards around Monarch Ingest development

Acceptance Criteria:
Markdown documentation that details contribution to the source code.

Example: The existing Monarch UI contribution guide is a great example https://github.com/monarch-initiative/monarch-ui/blob/master/CONTRIBUTING.md

[glass-ships]

This page should also include helpful tips, like monarch-initiative/monarch-ingest#243

It's really helpful in poetry to have your virtual environment get created in a consistent place, so that the IDE always know where to point, even if you might want to clobber the environment and start over.
We could add a suggestion in our contributing.md about using
poetry config virtualenvs.in-project true

[glass-ships]

This may be redundant now that we have some more detailed documentation for monarch ingest.
@kevinschaper do you have thoughts on this?