custom header documentation

docs/header-config.md

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+# Header Config
+
+With the header config, you can specify custom [HTTP Header](https://developer.mozilla.org/de/docs/Web/HTTP/Headers) for the responses of certain file types and paths.
+
+## Config
+
+You have to create a JSON file that serves as a config. The JSON must contain a `configs` array. For every entry, you can specify a certain path that must be matched as well as a file extension. You can use the `*` symbol to use the config entry for any path or filename. Note that the path option only matches the requested path from the start. Thatswhy you have to start with a `/` and can use paths like `/files/static/css`. The `headers` array includes a key-value pair of the actual header rule. The headers are not parsed so double check your spelling and test your site.
+
+The created JSON config has to be mounted into the container via a volume into `/config/headerConfig.json`. When this file does not exist inside the container, the header middleware will not be active.
+
+Example command to add to the docker run command:
+
+```
+-v /your/path/to/the/config/myConfig.json:/config/headerConfig.json
+```
+
+On startup, the container will log the found header rules.
+
+## Example headerConfig.json
+
+```json
+{
+  "configs": [
+    {
+      "path": "*",
+      "fileExtension": "html",
+      "headers": [
+        {
+          "key": "cache-control",
+          "value": "public, max-age=0, must-revalidate"
+        },
+        {
+          "key": "Strict-Transport-Security",
+          "value": "max-age=31536000; includeSubDomains;"
+        }
+      ]
+    },
+    {
+      "path": "*",
+      "fileExtension": "css",
+      "headers": [
+        {
+          "key": "cache-control",
+          "value": "public, max-age=31536000, immutable"
+        }
+      ]
+    },
+    {
+      "path": "/page-data",
+      "fileExtension": "json",
+      "headers": [
+        {
+          "key": "cache-control",
+          "value": "public, max-age=0, must-revalidate"
+        },
+        {
+          "key": "content-language",
+          "value": "en"
+        }
+      ]
+    },
+    {
+      "path": "/static/",
+      "fileExtension": "*",
+      "headers": [
+        {
+          "key": "cache-control",
+          "value": "public, max-age=31536000, immutable"
+        }
+      ]
+    }
+  ]
+}
+```