Update README.md (#18936)
Update README.md (#18936)

* Update README.md 

enhance folder structure guidance

* Update README.md

* minor update to "preview" and "stable" section

* add further clarification to service group folder

* minor update of the wording

* Update README.md

add future description of service group folder structure.

* Update README.md

some minor updates

* Update README.md

Co-authored-by: Mike Kistler <f76d68e6a0386bfb4a8a08e90c7abcedcd090fe6@microsoft.com>

* Update README.md

* Update README.md

Co-authored-by: Mike Kistler <f76d68e6a0386bfb4a8a08e90c7abcedcd090fe6@microsoft.com>
solankisamir/azure-rest-api-specs,solankisamir/azure-rest-api-specs,naveedaz/azure-rest-api-specs,naveedaz/azure-rest-api-specs