stac-utils developer guide

[gadomski]

I've created a stac-utils developer guide: http://stac-utils.github.io/developer-guide/. Source code is here: https://github.com/stac-utils/developer-guide.

My theory is that the developer guide can be a single point of reference for best practices, policies, etc for the stac-utils Github organization. Right now it includes some recommended issue and pull request templates, some words on semantic versioning and dependency management, and some de-facto policies around owners, maintainers, etc.

Contributions and suggestions always welcome!