Update README about Laravel version

[hms5232]

Now, the badge will auto renew depend on composer.json.

How and why? Every Laravel package should implement and extend the Illuminate\Support\ServiceProvider which include in illuminate/support. The versions of those packages from illuminate vendor are following the version of Laravel framework. Therefor, we can directly show the version of illuminate/support that in composer.json to indicate supported version in this package.

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If PR is accepted and you don't mind, could you add a hacktoberfest-accepted label to help me with hacktoberfest? Thanks!