Add changelog for 4.6.0
Add changelog for 4.6.0

Co-authored-by: Simon Cross <403f597505b2ab3c7891c3e345ef807ea33f2a9f@gmail.com>
Co-authored-by: Boxi Li <3238121b9c59c7207bf1fec821af6ae73f13ac08@gmail.com>

cgranade/qutip,qutip/qutip,cgranade/qutip,qutip/qutip