ext: Update ext/sst/README.md for v22.1 release

Change-Id: I3221bbc4e4bf1d7a2bd7516873c7bfcf08bebde5
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/65924
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
1 file changed