L2SS-905 added readme with instructions on how to support 2 subracks

Closes L2SS-905

assigned to @snijder

enabled an automatic merge when the pipeline for 4d1164c3 succeeds

changed title from L2SS-905 added readm with instructions on how to support 2 subracks to L2SS-905 added readme with instructions on how to support 2 subracks

mentioned in commit 437f5d98

merged