New thread guidelines

[wilzbach]

Fixes #69 by allowing to set a custom url for a thread that display the guidelines for this thread.
I thought supplying a URL is more flexible than hard-coding HTML and it's inspired after Github contribution guidelines:

https://github.com/blog/1184-contributing-guidelines

I think if it's an URL not all people won't bother reading anyway, so it's a less intrusive addition than putting some guidelines directly on the page. However also not as effective ;-)

Btw I had some troubles running DFeed, because for non-sudo user it isn't allowed to use ports lower than 1024. Hence as an improvement for other newcomers, the first commit sets the default port to 8080

[CyberShadow]

If you want to change this then you should also update the README. Ideally it would be in its own PR.