Add a reminder of Pre-Depends policy to §7.2 "Binary Dependencies"

Add a reminder of Pre-Depends policy to §7.2 "Binary Dependencies"

Section 3.5 says:

 You should not specify a Pre-Depends entry for a package before this
 has been discussed on the debian-devel mailing list and a consensus
 about doing that has been reached.

Add a reminder of this to section 7.2, where the semantics of
Pre-Depends are discussed in more detail.

Requested-by: Mattias Ellert <mattias.ellert@fysast.uu.se>
Fixes: http://bugs.debian.org/599944
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>