maint: Adds HTTP headers example to values.yaml (#299)

## Which problem is this PR solving?

Updates the example helm-chart to show how to configure additional HTTP
headers to be extracted.

- Implemented in
honeycombio/honeycomb-network-agent#277

## Short description of the changes
- Update example values.yaml with commented out config that shows how to
set additional HTTP headers to extract

## How to verify that this has the expected result
The values.yaml show how to set additional headers to extract.

charts/network-agent/values.yaml

@@ -27,3 +27,6 @@ resources: {}
 extraEnvVars:
   # - name: ENV_VAR
   #   value: value
+  # Configures the list of HTTP headers to be recorded from requests/responses.
+  # - name: HTTP_HEADERS  
+  #   value: header1,header2