Add Contributing Guidelines for project

This project may benefit from a CONTRIBUTING.rst file showing how to get setup with development and expectations for contributors. I would probably base it off https://github.com/marshmallow-code/marshmallow/blob/dev/CONTRIBUTING.rst other than the stuff specific to marshmallow.

Would we be open to a PR for this?

Start by reading the marshmallow CONTRIBUTING.rst file linked in the issue and inspect the flask-apispec project for details that need to differ. Add a project-specific CONTRIBUTING.rst covering development setup and contributor expectations; it is done when those guidelines are documented in the new file.

Written by the indexing model from the issue text.