misc: Add release notes for 20.1
misc: Add release notes for 20.1

Change-Id: I011ff987e222326dd7f0787c41043578b52b236a
Signed-off-by: Jason Lowe-Power <68c46a606457643eab92053c1c05574abb26f861@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35375
Maintainer: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Reviewed-by: Giacomo Travaglini <8eef13a4f970e2e9372c8d26ad0737b760f99c02@arm.com>
Reviewed-by: Matthew Poremba <f1eb50f3368ce197c52b19f7fdb5aa7a278f15fd@amd.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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