minor symfony#57485 [Mailer] [Infobip] Document the usage of custom headers (ndousson)

This PR was merged into the 7.2 branch.

Discussion
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[Mailer] [Infobip] Document the usage of custom headers

| Q             | A
| ------------- | ---
| Branch?       | 7.2
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Issues        | -
| License       | MIT

See symfony#57424

Commits
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fc4403d [Mailer][Infobip] Document the usage of custom headers

Author: fabpot

src/Symfony/Component/Mailer/Bridge/Infobip/README.md

@@ -12,8 +12,25 @@ MAILER_DSN=infobip+api://KEY@BASE_URL
 MAILER_DSN=infobip+smtp://KEY@default
 ```
 
+Custom Headers
+--------------
+
+This transport supports the following custom headers:
+
+| Header                         | Type    | Description                                                                             |
+|--------------------------------|---------|-----------------------------------------------------------------------------------------|
+| `X-Infobip-IntermediateReport` | boolean | The real-time Intermediate delivery report that will be sent on your callback server.   |
+| `X-Infobip-NotifyUrl`          | string  | The URL on your callback server on which the Delivery report will be sent.              |
+| `X-Infobip-NotifyContentType`  | string  | Preferred Delivery report content type. Can be application/json or application/xml.     |
+| `X-Infobip-MessageId`          | string  | The ID that uniquely identifies the message sent to a recipient.                        |
+| `X-Infobip-Track`              | boolean | Enable or disable open and click tracking.                                              |
+| `X-Infobip-TrackingUrl`        | string  | The URL on your callback server on which the open and click notifications will be sent. |
+| `X-Infobip-TrackClicks`        | boolean | Enable or disable track click feature..                                                 |
+| `X-Infobip-TrackOpens`         | boolean | Enable or disable open click feature.                                                   |
+
 Resources
 ---------
+
 * [Infobip Api Docs](https://www.infobip.com/docs/api#channels/email)
 * [Contributing](https://symfony.com/doc/current/contributing/index.html)
 * [Report issues](https://github.com/symfony/symfony/issues) and