add CONTRIBUTING guidelines document

Simply create a document called CONTRIBUTING.md in the root of the repository, use it to describe the preferred issue reporting workflow, and

Make sure to emphasize the importance of submitting issues for anything that the user does not understand.

[Feel free to draw inspiration from here](https://github.com/microsoft/vscode/blob/cf4c4a0ece70d221d907a2bc0042cc164f73bb0c/CONTRIBUTING.md)

Create CONTRIBUTING.md at the repository root. First review the linked VS Code CONTRIBUTING.md for structure, then document the preferred issue-reporting workflow and emphasize reporting anything users do not understand; done means the file contains both requirements and is committed at the root.

Written by the indexing model from the issue text.