Add badge for weekly cron

[WilliamJamieson]

This PR adds a badge to the readme showing the status of the weekly cron. I find this useful when checking if all is well.

Checklist

• Added entry in CHANGES.rst under the corresponding subsection
• updated relevant tests
• updated relevant documentation
• Passed romancal regression testing on Jenkins / PLWishMaster. Link: https://plwishmaster.stsci.edu:8081/job/RT/job/romancal/XXX/

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (4ed3443) 96.67% compared to head (c93b0d4) 96.67%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #264   +/-   ##
=======================================
  Coverage   96.67%   96.67%           
=======================================
  Files          28       28           
  Lines        2406     2406           
=======================================
  Hits         2326     2326           
  Misses         80       80           

see 2 files with indirect coverage changes

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[PaulHuwe]

LGTM