RFC: Adding a SECURITY.md document to the Binutils

[Eli Zaretskii]

> Date: Fri, 7 Apr 2023 09:42:25 +0100
> Cc: siddhesh@gotplt.org, "gdb@sourceware.org" <gdb@sourceware.org>
> From: Nick Clifton via Gdb <gdb@sourceware.org>
> 
>    Thoughts ?  Comments ?

Thanks.  A minor nit: does it have to be a .md file?  Can it be a .txt
file instead?