Add documentation for checks to README

Adds a description of the new checks functionality (lhcb-dpa/analysis-productions/lbapcommon!21 (merged), lhcb-dpa/analysis-productions/lbapcommon!24 (merged), lhcb-dpa/analysis-productions/lbapcommon!27 (merged), lhcb-dpa/analysis-productions/lbapcommon!29 (merged)) to the main README. It's probably best to merge only after lhcb-dpa/analysis-productions/lbaplocal!21 (merged) since until that is merged the checks will not be functioning.

DPA task https://gitlab.cern.ch/lhcb-dpa/project/-/issues/146 on documentation.