[docs]: Add TODO for subject templates

Author: EthanKim8683

exporter/natscoreexporter/README.md

@@ -7,6 +7,8 @@ NATS Core exporter exports logs, metrics and traces to [NATS Core](https://docs.
 
 ## Configuration
 
+**TODO: With regards to subjects, `text/template` can only call methods with no arguments. This won't work for `Resource`s, for example, since `Resource` attributes are only accessible using the `Get(string)` method. I need to find or design an alternative that supports this.**
+
 The following configuration options are supported:
 
 - `endpoint` (default = nats://localhost:4222): The NATS server URL.