Update readme (#660)

README.md

@@ -209,6 +209,37 @@ Used for proecessing flexforms.
 ### RootSitesProcessor
 Render your all headless sites configuration for your frontend application.
 
+## Configuration
+### Available Settings:
+- **Not Enabled**: Headless mode is deactivated.
+- **Mixed Mode**: Fluid and headless operate concurrently.
+- **Fully Headless Mode**: Headless mode is fully activated.
+
+To set up headless mode, utilize the site configuration flag as shown below:
+
+```yaml
+'headless': 0|1|2
+```
+
+While the legacy flag (true|false) is still recognized, transitioning to the integer notation is recommended.
+
+### Options:
+- **0** (formerly: false) = headless mode is deactivated for the site within the TYPO3 instance.
+- **1** (formerly: true) = headless mode is fully activated for the site within the TYPO3 instance.
+- **2** = mixed mode headless is activated (both fluid & json API are accessible within a single site in the TYPO3 instance).
+
+Options **0** (formerly: false) or **1** (formerly: true) inform the extension to either fully disable or enable headless mode for a particular site.
+
+### To Enable Mixed Mode:
+For a chosen site in TYPO3, follow these steps:
+- In the typoscript template for the site, load the "Headless - Mixed mode JSON response" setup file instead of the default headless one.
+- Set `headless` flag to a value of `2` in the site configuration file or configure the flag via editor in the Site's management backend.
+
+The mixed mode flag (value of `2`) instructs the EXT:headless extension to additionally check for the `Accept` header with a value of `application/json` when processing requests to the particular site in the TYPO3 instance.
+
+- In cases where a request lacks the `Accept` header or `Accept` has a different value than `application/json`, TYPO3 will respond with HTML content (standard TYPO3's response).
+- In cases where a request's header `Accept` matches the value of `application/json`, TYPO3 will respond with a JSON response.
+
 ## Development
 Development for this extension is happening as part of the TYPO3 PWA initiative, see https://typo3.org/community/teams/typo3-development/initiatives/pwa/
 If you have any questions, join #initiative-headless-pwa Slack channel.