Add a RISC-V NEWS entry for 2.28

[Tristan Gingold]

> On 04 Mar 2017, at 02:28, Palmer Dabbelt <palmer@dabbelt.com> wrote:
> 
> It looks like I forgot to add a blurb about RISC-V support to the 2.28
> binutils/NEWS section.  Is it too late to do this?

Yes for 2.28, but you can still add it in case of a 2.28.1 release.