run weekly R CMD check (#72)

Author: cjyetman

.github/workflows/R-CMD-check.yaml

@@ -9,6 +9,8 @@ on:
     branches: [main, master]
   pull_request:
     branches: [main, master]
+  schedule:
+    - cron: '0 8 * * MON'
 
 name: R-CMD-check