Clarify runtime variable interpolation in README

Added a note explaining that certain runtime variables like BUILDKITE_RETRY_COUNT may not expand reliably due to interpolation timing. Referenced a link to documentation on runtime variable interpolation for further info.

Author: JoeColeman95

README.md

@@ -173,6 +173,8 @@ When set to true, it will activate interpolation of variables in the elements of
 
 ⚠️ Important: this is considered an unsafe option as the most compatible way to achieve this is to run the strings through `eval` which could lead to arbitrary code execution or information leaking if you don't have complete control of the pipeline.
 
+Note: Runtime variables like `BUILDKITE_RETRY_COUNT` may not expand reliably as Buildkite interpolates pipeline YAML at upload time, before this plugin runs. In cases such as this, consider using [Runtime variable interpolation](https://buildkite.com/docs/pipelines/configure/environment-variables#runtime-variable-interpolation)
+
 ### `ignore-missing` (boolean)
 
 If set to `true`, it will ignore errors caused when calling `buildkite-agent artifact` to prevent failures if you expect artifacts not to be present in some situations. When using the `compressed` property, it will ignore compressing the artifacts that are not present.