Add a RISC-V NEWS entry for 2.28

[Palmer Dabbelt]

On Mon, 06 Mar 2017 00:46:14 PST (-0800), Tristan Gingold wrote:
>
>> 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.

OK, makes sense.  Is this the right place to put it?