Feature request: update mailjet's doc with example

[badpenguin]

I was worried to patch the library for supporting URLTags, customID and CustomCampaign, etc. But it seems that it works by just putting them in Globals instead of Messages. So maybe you want to update mailjet's doc about this:

    email.esp_extra = {
        "Globals": {
            "CustomID": "123456",
            "CustomCampaign": "validation_message",
            "URLTags": "utm_source=mailjet&utm_medium=email&utm_campaign=validation&utm_content=test1&utm_term=v1",
        }
    }

[medmunds]

To set a Mailjet CustomCampaign, use Anymail's tags option.

Although you can use a Mailjet CustomID as in your example, a more portable option is Anymail's message_id. Anymail supports this for Mailjet and all other ESPs. (For Mailjet, message_id maps to their “legacy” MessageID, which is also provided to webhooks.)

Using other Mailjet features not directly supported by Anymail is documented here: https://anymail.dev/en/stable/esps/mailjet/#esp-extra-support. If you'd like to update the example to include URLTags, a PR is welcome.