Update posts.php

Added functionality to display the "read more" button only, when there is really more.

Double-check your Markdown File.
The Button will only be hidden, when there is no line 10. (Because the "intro" is the first paragraph [line 8] in dropplets)

So a post without a button should look like this:

1 #Title
2 - Author
3 - Twitter
4 - Date
5 - Topic
6 - Status
7 *empty line*
8 *post intro*
9 *empty line (my editor always adds this)*