[chore][release.md] mention release blockers (#11908)

#### Description
Mention release blockers should be addressed before the release.

docs/release.md

@@ -21,6 +21,8 @@ It is possible that a core approver isn't a contrib approver. In that case, the
 To ensure the rest of the community is informed about the release and can properly help the release manager, the release manager should open a thread on the #otel-collector-dev CNCF Slack channel and provide updates there.
 The thread should be shared with all Collector leads (core and contrib approvers and maintainers).
 
+Before the release, make sure there are no open release blockers in [core](https://github.com/open-telemetry/opentelemetry-collector/labels/release%3Ablocker), [contrib](https://github.com/open-telemetry/opentelemetry-collector-contrib/labels/release%3Ablocker) and [releases](https://github.com/open-telemetry/opentelemetry-collector-releases/labels/release%3Ablocker) repos.
+
 ## Releasing opentelemetry-collector
 
 1. Update Contrib to use the latest in development version of Core by running `make update-otel` in Contrib root directory. This is to ensure that the latest core does not break contrib in any way. If it results in any changes, submit a PR to Contrib. If you are unable to run `make update-otel`, it is possible to skip this step and resolve conflicts with Contrib after Core is released, but this is generally inadvisable.