Instructions for header case

Author: mendhak

README.md

@@ -32,6 +32,7 @@ This image is executed as non root by default and is fully compliant with Kubern
 - [Include environment variables in the response](#include-environment-variables-in-the-response)
 - [Configuring CORS policy](#setting-corscross-origin-resource-sharing-headers-in-the-response) 
 - [Client certificate details (mTLS) in the response](#client-certificate-details-mtls-in-the-response)
+- [Preserve the case of headers in response body](#preserve-the-case-of-headers-in-response-body)
 - [Prometheus Metrics](#prometheus-metrics)
 - [Screenshots](#screenshots)
 - [Building](#building)
@@ -278,6 +279,14 @@ If you browse to https://localhost:8443/ in Firefox, you won't get prompted to s
 openssl pkcs12 -export -in cert.pem -inkey privkey.pem -out certpkcs12.pfx
 ```
 
+## Preserve the case of headers in response body
+
+By default, the headers in the response body are lowercased. To attempt to preserve the case of headers in the response body, set the environment variable `PRESERVE_HEADER_CASE` to true.
+
+```bash
+docker run -e PRESERVE_HEADER_CASE=true -p 8080:8080 -p 8443:8443 --rm -t mendhak/http-https-echo:33
+```
+
 ## Prometheus Metrics
 
 To expose http performance metrics, set the `PROMETHEUS_ENABLED` environment variable to true, the metrics will be available at `/metrics`. This uses the [`express-prom-bundle`](https://github.com/jochen-schweizer/express-prom-bundle) middleware