[chore] add clarification about the usage of ready to merge label (open-telemetry#45474)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description

The usage of the `ready to merge` label is not documented. This PR adds
the clarification of when it should be added as part of the PR review
guidelines to ensure proper usage within the project.

FYI @open-telemetry/collector-contrib-approvers
@open-telemetry/collector-contrib-triagers

<!-- Issue number (e.g. open-telemetry#1234) or full URL to issue, if applicable. -->
#### Link to tracking issue
Fixes ~

<!--Describe what testing was performed and which tests were added.-->
#### Testing
~

<!--Describe the documentation added.-->
#### Documentation
Updated

<!--Please delete paragraphs that you did not use before submitting.-->

Signed-off-by: ChrsMark <chrismarkou92@gmail.com>

Author: ChrsMark

README.md

@@ -166,4 +166,7 @@ is a maintainer they are responsible for merging the PR as well.
 The facilitator is not required to perform a thorough review, but they are encouraged to
 enforce Collector best practices and consistency across the codebase and component
 behavior. The facilitators will typically rely on codeowner's detailed review of the code
-when making the final approval decision. 
+when making the final approval decision.
+
+Marking the PR with the `ready to merge` label should only happen (by triagers/approvers/maintainers)
+once there is at least one approval from an approver, as per the description above.