docs: Add —-semantic-layer flag documentation to README

[kiwamizamurai]

closes #163

Summary

This PR adds the missing documentation for the --semantic-layer flag in the README's usage section, as requested in #163.

Changes

• Added documentation for --semantic-layer flag in the deprecations command options list
• Included a clear description of what the flag does (migrates semantic models and metrics into dbt model definitions)
• Noted the incompatibility with --include-packages option

Motivation

The --semantic-layer flag was implemented and functional but not documented in the README, making it difficult for users to discover and use this feature.

[kiwamizamurai]

@b-per please review

[b-per]

Thanks. We will wait to release compatibility with the new SL both in Fusion and Core before documenting the flag. We might also be able to update the wording once the announcement of the new spec is done.

[kiwamizamurai]

@b-per

I see.
I tested it locally, and it seems the new SL is coming soon. I’m really looking forward to it!
I’ll take a look at the issues to see if there’s anything else I can contribute.
Thank you for all your work on dbt!