Current version number

[twocs]

The Readme says to use composer with {version} but it wasn't easily apparent to me what was the recommended or current version. Looking at the github.com page, the only version notation is "Version 5.0.2" that appears in commit messages. Only after installation did

Version doesn't appear in the composer.json (it's not recommended to appear there). But as mentioned in the Readme, version needs to appear in our require section:

    "require": {
        "campaignmonitor/createsend-php": "{version}"
    }
}```

I'd propose that this part of the Readme should briefly mention how to find that version number. Do you recommend getting the latest release? Currently we can see that the version is v6.0.1 via this link:
https://github.com/campaignmonitor/createsend-php/releases/latest