Removed suggesting of using the comments system from the introduction

The introduction still contained reference to the old "add a comment" feature. I have adjusted this paragraph to point to the book's github page for now.

I don't know whether this is ideal though; on the one hand, pushing rst on someone just starting out with django may be off-putting, but on the other it introduces GitHub (pretty much vital for django folks) and reinforces the idea of continual improvement, community and encouraging involvement.