Update README with a new section about OpenAPI 3.0 to avoid more questions #2192
Description
Problem statement
It's not easy to find out if the maintainers of this project intend to support OpenAPI 3.0.
That leads to issues like #2180 and #1122, and repeated questions in Slack and elsewhere.
Proposed Solution
Add a new section to the README that explains the situation (e.g. based on @casualjim's comment on #1122) and offers suggestions for alternative approaches.