[fileexporter] Clarify writable directory requirements (#16101)

[fileexporter] Clarify writable directory req

As the [official image is made](https://github.com/open-telemetry/opentelemetry-collector-releases/blob/main/distributions/otelcol-contrib/Dockerfile) using the special `from scratch` directive, the official image does not have a writable directory by default. This clarifies the docs around that to reduce the number of issues.
This came up in #13630

exporter/fileexporter/README.md

@@ -18,6 +18,8 @@ Exporter supports the following features：
 Please note that there is no guarantee that exact field names will remain stable.
 This intended for primarily for debugging Collector without setting up backends.
 
+The official [opentelemetry-collector-contrib container](https://hub.docker.com/r/otel/opentelemetry-collector-contrib/tags#!) does not have a writable filesystem by default since it's built using the special `from scratch` layer. As such, you will need to create a writable directory for the path, potentially by creating writable volumes or creating a custom image.
+
 ## Getting Started
 
 The following settings are required:
@@ -89,4 +91,4 @@ exporters:
 
 [alpha]:https://github.com/open-telemetry/opentelemetry-collector#alpha
 [contrib]:https://github.com/open-telemetry/opentelemetry-collector-releases/tree/main/distributions/otelcol-contrib
-[core]:https://github.com/open-telemetry/opentelemetry-collector-releases/tree/main/distributions/otelcol
+[core]:https://github.com/open-telemetry/opentelemetry-collector-releases/tree/main/distributions/otelcol