docs: Add a note on full CI run in contributing guide (vllm-project#13646)

Author: terrytangyuan

docs/source/contributing/overview.md

@@ -145,6 +145,9 @@ review process:
 - Please respond to all comments within a reasonable time frame. If a comment
   isn't clear or you disagree with a suggestion, feel free to ask for
   clarification or discuss the suggestion.
+- Note that not all CI checks will be executed due to limited computational
+  resources. The reviewer will add `ready` label to the PR when the PR is
+  ready to merge or a full CI run is needed.
 
 ## Thank You