add cross sections for dark Higgs models

[mcremone]

[SiewYan]

Hi, the darkhiggs cross section is computed by hadronizer and can be found here:

https://cernbox.cern.ch/index.php/s/C5YBz7a7tBFZcnk

[mcremone]

@sudhaahuja @sundleeb would you mind including these?

[sudhaahuja]

Ok, will include them