cron docs

reference/specs/bindings/cron.md

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+# Cron Binding Spec
+
+```yaml
+apiVersion: dapr.io/v1alpha1
+kind: Component
+metadata:
+  name: <NAME>
+  namespace: <NAMESPACE>
+spec:
+  type: bindings.cron
+  metadata:
+  - name: schedule
+    value: "@every 15m" # valid cron schedule  
+```
+
+## Schedule Format 
+
+The Dapr cron binding supports following formats: 
+
+| Character |	Descriptor        | Acceptable values                             |
+|:---------:|-------------------|-----------------------------------------------|
+| 1	        | Second	          | 0 to 59, or *                                 |
+| 2	        | Minute	          | 0 to 59, or *                                 |
+| 3	        | Hour	            | 0 to 23, or * (UTC)                           |
+| 4	        | Day of the month	| 1 to 31, or *                                 |
+| 5	        | Month	            | 1 to 12, or *                                 |
+| 6	        | Day of the week	  | 0 to 7 (where 0 and 7 represent Sunday), or * |
+
+For example: 
+
+* `30 * * * * *` - every 30 seconds
+* `0 15 * * * *` - every 15 minutes
+* `0 30 3-6,20-23 * * *` - every hour on the half hour in the range 3-6am, 8-11pm
+* `CRON_TZ=America/New_York 0 0 30 04 * * *` - every day at 4:30am New York time
+
+> You can learn more about cron and the supported formats [here](https://en.wikipedia.org/wiki/Cron)
+
+For ease of use, the Dapr cron binding also supports few shortcuts:
+
+* `@every 15s` where `s` is seconds, `m` minutes, and `h` hours
+* `@daily` or `@hourly` which runs at that period from the time the binding is initialized  
+