📄 Create collaborative, inviting `CONTRIBUTING.md`

[JFWooten4]

Create a contributing guide based on industry / open source best practices. 💡
Should explain the dual-licensing nuances for images v. other documentation.

[JFWooten4]

Moving here so that we can follow a global framework, streamlining collaboration for all repos.

[JFWooten4]

Using the knowledge from implementing this in the Stellar Docs, but therefore taking a while longer.