Update DESIGN.md

docs/DESIGN.md

@@ -16,11 +16,12 @@ An app is simply a `git` repository with some additional files on the top level,
 
 ### `Procfile` format
 
-`piku` recognizes three kinds of process declarations in the `Procfile`:
+`piku` recognizes five kinds of process declarations in the `Procfile`:
 
 * `wsgi` workers, in the format `dotted.module:entry_point` (Python-only)
 * `web` workers, which can be anything that honors the `PORT` environment variable
 * `static` workers, which simply mount the first argument as the root static path
+* `cron` workers, which require a simplified `cron` expression preceding the command to be run (e.g. `cron: */5 * * * * python batch.py` to run a batch every 5 minutes)
 * `worker` processes, which are standalone workers and can have arbitrary names
 
 So a Python application could have a `Procfile` like such:
@@ -29,6 +30,8 @@ So a Python application could have a `Procfile` like such:
 wsgi: module.submodule:app
 worker: python long_running_script.py 
 fetcher: python fetcher.py
+# Simple cron expression: minute [0-59], hour [0-23], day [0-31], month [1-12], weekday [1-7] (starting Monday, no ranges allowed on any field)
+cron: 0 0 * * * python midnight_cleanup.py
 ```
 
 ...whereas a generic app would be: