Add example of use

src/handlers/jobs.rs

@@ -4,7 +4,29 @@
 
 // The metadata is a serde_json::Value
 // Please refer to https://docs.rs/serde_json/latest/serde_json/value/fn.from_value.html
-// on how to interpret it as an instance of type T, implementing Deserialize.
+// on how to interpret it as an instance of type T, implementing Serialize/Deserialize.
+
+// For example, if we want to sends a Zulip message every Friday at 11:30am ET into #t-release 
+// with a @T-release meeting! content, we should create some Job like:
+//    
+//    #[derive(Serialize, Deserialize)]
+//    struct ZulipMetadata {
+//      pub message: String
+//    }
+//    
+//    let metadata = serde_json::value::to_value(ZulipMetadata {
+//      message: "@T-release meeting!".to_string()
+//     }).unwrap();
+// 
+//    Job {
+//      name: "send_zulip_message",
+//      expected_time: "2022-09-30T11:30:00+10:00",
+//      frequency: Some(7),
+//      frequency_unit: Some(FrequencyUnit::Days),
+//      metadata: metadata
+//    }
+// 
+// ... and add the corresponding "send_zulip_message" handler.
 
 pub async fn handle_job(name: &String, metadata: &serde_json::Value) -> anyhow::Result<()> {
     match name {