Add $entry->isSlowQuery() in documentation.

[MishukAdhikari]

As this PR has been merged, I think it's better to add the method in documentation so it will be easier for developers to find it.

    Telescope::filter(function (IncomingEntry $entry) {
        if ($this->app->environment('local')) {
            return true;
        }

        return $entry->isReportableException() ||
            $entry->isFailedJob() ||
            $entry->isScheduledTask() ||
            $entry->isSlowQuery() ||
            $entry->hasMonitoredTag();
    });

[MishukAdhikari]

https://github.com/laravel/telescope/releases/tag/v4.6.7

This has been released already.

Thanks to Laravel Team.