Add a note about OTEPs moving to the Specification. (#268)

Co-authored-by: Trask Stalnaker <[email protected]>
Co-authored-by: Robert Pająk <[email protected]>

README.md

@@ -3,6 +3,11 @@
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+> [!WARNING]  
+> OTEPs have been moved to the [Specification](https://github.com/open-telemetry/opentelemetry-specification/tree/main/oteps/)
+> repository. This repository has been preserved for reference purposes.
+> Please otherwise refer to the Specification.
+
 ## Evolving OpenTelemetry at the speed of Markdown
 
 OpenTelemetry uses an "OTEP" (similar to a RFC) process for proposing changes to the [OpenTelemetry specification](https://github.com/open-telemetry/opentelemetry-specification).