[Docs] Add general information about experimental use to the README

[jsquire]

Summary

As the release strategy has changed to an approach of "always stable versions" that differentiate features between stable and experimental, it has been noted that our documentation does not discuss [Experimental] use nor how it should be suppressed beyond a brief mention specifically associated with the assistants feature.

To ensure that developers understand how to opt-into experimental features and suppress warnings, we should add a section prominently in the README.

Scope

• Add a new section to the README following Prerequisites but before Using the client library which discusses the distinction between stable and experimental features, use of the [Experimental] attribute, and techniques for suppressing the warnings.

• Ensure that the section also touches on "warnings as errors" and that if it enabled in your project, using experimental features may cause build failures.

• Details about what experimental codes are used and what features they are associated with should be included.

• The process for adding new experimental codes should be updated to include ensuring the README content includes the new code information.

References and resources

• [QUESTION] Usage of Experimental attribute (#508)
• Feature Lifecycle in the OpenAI .NET Client Library

[jsquire]

//cc: @scottaddie, @achandmsft