[review] Add a NEWS entry for multithreaded symbol loading

[Eli Zaretskii]

> Date: Wed, 27 Nov 2019 13:03:13 -0500
> From: "Tom Tromey (Code Review)" <gerrit@gnutoolchain-gerrit.osci.io>
> 
> Tom Tromey has posted comments on this change.
> 
> Change URL: https://gnutoolchain-gerrit.osci.io/r/c/binutils-gdb/+/725
> ......................................................................
> 
> 
> Patch Set 1: Code-Review+1
> 
> Looks reasonable to me, but Eli should review.

I already did.