misc: Minor updates to CONTRIBUTING.md
misc: Minor updates to CONTRIBUTING.md

This brings the file slightly more up to date

Change-Id: I1ed3300ec3c4980ed22c6a6fb950fa724897906b
Signed-off-by: Jason Lowe-Power <68c46a606457643eab92053c1c05574abb26f861@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36255
Reviewed-by: Daniel Carvalho <91779fc734d358d9575ec4910de29bfcbcaf1b22@yahoo.com.br>
Reviewed-by: Matt Sinclair <d2406197fb1bedd1e23c70c50c510ff0e9ab0f5b@gmail.com>
Maintainer: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5