Make it clear that the library is not stable

[NickUfer]

When developers open the main page and read the README of this project for the first time everything, seems like this library is stable, which is not the case.

Please add a note at the top which makes it clear that this project is not even in beta yet (according to #1678).

[cijothomas]

https://github.com/open-telemetry/opentelemetry-rust?tab=readme-ov-file#project-status -- is this sufficient?
Is there anything in crates.io to mark stability status?

[cijothomas]

https://github.com/open-telemetry/opentelemetry-rust/blob/main/VERSIONING.md#policy it is also documented that the repo follows semver2.0.
And no crates have reached 1.0 - which is a reasonable way of communicating that breaking changes are possible.
If there are other ideas, happy to explore. Closing this issue for now.