website: Add a gem5-20 release post

Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Change-Id: I3d8164ed54f4a42192804bf45857eb9eccfad474
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/29312
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby R. Bruce <bbruce@ucdavis.edu>
1 file changed