Add note that Entries plugin requires 'index' Page

[cylkee]

After I couple of hours of testing, head-scratching, head-banging, I noticed the comment here https://github.com/symphonists/rest_api/blob/30ad34a934f79ab1a91628edd88d3291efb0e3f7/plugins/entries/rest.entries.php#L88

I was testing in a new installation without any pages yet, wondering why a 404 was being thrown, that wasn't an API formatted 404.