Add lacking Compute Engine link to the Services section (#103)

Signed-off-by: Koichi Shiraishi <zchee.io@gmail.com>
Co-authored-by: Armin Ruech <armin.ruech@dynatrace.com>
Co-authored-by: Josh Suereth <joshuasuereth@google.com>

specification/resource/semantic_conventions/cloud_provider/gcp/README.md

@@ -10,5 +10,6 @@ provider (like account ID, operating system, etc), it belongs in the parent
 ## Services
 
 - [Cloud Run](./cloud_run.md)
+- [Compute Engine](./gce.md)
 
 [DocumentStatus]: https://github.com/open-telemetry/opentelemetry-specification/blob/v1.21.0/specification/document-status.md