Add docstring for cron branches (#30)

Albrja/mic-6030/doc-cron-branches

Add docstring for how to handle cron branches in Jenkins
- *Category*: CI
- *JIRA issue*: https://jira.ihme.washington.edu/browse/MIC-6030

Author: albrja

Jenkinsfile

@@ -28,7 +28,12 @@ Note that updating the shared repo will take affect on the next pipeline invocat
 library("vivarium_build_utils@${get_vbu_version()}")
 
 reusable_pipeline(
-    scheduled_branches: ["main"], 
+    scheduled_branches: [
+      // Add additional branches for cron jobs below.
+      // Note: Newly added branches will require a manual trigger of the first build 
+      // for subsequent scheduled builds to run.
+      "main"
+    ],
     test_types: ["unit"], 
     requires_slurm: false, 
 )