[Opm] Release 2020.04

[Joakim Hove]

Hello;


I would like to remind all relevant parties about the upcoming 2020.04 
release. The release branches will be created on wednesday 22.nd of 
April - don't miss the train; and please mark open PR's which you want 
to get in with "2020.04 Release" label.

Timeline: https://github.com/OPM/opm-simulators/issues/2519


Joakim