Add coverage badge

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Code Coverage Summary

Filename                    Stmts    Miss  Cover    Missing
------------------------  -------  ------  -------  ---------
R/synthetic_cdisc_data.R       61      61  0.00%    15-137
R/zzz.R                        10       4  60.00%   6-9
TOTAL                          71      65  8.45%

Diff against main

Filename    Stmts    Miss    Cover
----------  -------  ------  -------
TOTAL       -        -       -

Results for commit: 9c876e10bd850f4a152f85fa6b6466cf725cd4f6

Minimum allowed coverage is 80%

:recycle: This comment has been updated with latest results