Added acknowledgements section for talks mentioning "astropy-powered" logo

This adds a section to the acknowledgements that encourages the use of the "astropy-powered" version of the logo for use in talks/presentations.

@astrofrog - I don't think too many people watch this space, so should I post a note on astropy-dev asking what people think of this?  Or should we go ahead and merge it and just do an advisory there bringing it to peoples' attention?