Adds directions for updating the changelog (and some justification for why it exists and why you should add to it)
This should fix the problem reported in #853. I also changed a few log.warning -> warnings.warn in the convolve module, and moved the small bit of warnings documentation to a more prominent place.  While I was in there I added an example to the astropy.io.fits.diff documentation since it was looking a little bare without the warnings documentation before it.