README: Why users cannot create issues directly

[nikitabobko]

Users are not allowed to create Issues directly in this repository - we ask that you create a Discussion first.

Users can't create issues directly because:

• Users submit too many duplicates without prior search
• A lot of user issues are misunderstandings, environmental problems, or configuration errors by the users themselves
• Few people can formulate proper bug reports
• Even fewer people can formulate proper, actionable feature requests that align with other existing/planned features and the overall AeroSpace mental model

Any Discussion which clearly identifies a problem and can be confirmed or reproduced will be converted to an Issue by maintainers.

This whole pattern makes it easier for maintainers or contributors to find issues to work on. Issues are like a publicly observable maintainers' inbox. We want to keep this inbox tidy and clean.

We are in transitioning phase, so there are some still unverified issues, which will be closed or moved to Discussions. But issues marked with Triaged label are definetely verified.

For more details, see CONTRIBUTING.md