Add README for cmd/samples/recipes/timer

Signed-off-by: Diana Zawadzki <[email protected]>

Author: zawadzkidiana

cmd/samples/recipes/timer/README.md

@@ -0,0 +1,84 @@
+# Timer Sample
+
+This sample demonstrates **timer-based notifications** - running a long operation with a timeout that triggers an alert if processing takes too long.
+
+> **Looking for a visual guide?** See [new_samples/hello_world](../../../../new_samples/hello_world/) for a step-by-step tutorial with screenshots.
+
+## How It Works
+
+```
+┌──────────────────────────────────────────────────────────────┐
+│                      Workflow Start                          │
+└──────────────────────────────────────────────────────────────┘
+                              │
+              ┌───────────────┴───────────────┐
+              ▼                               ▼
+   ┌─────────────────────┐         ┌─────────────────────┐
+   │ orderProcessing()   │         │ Timer (3 seconds)   │
+   │ (random 0-10 sec)   │         │                     │
+   └─────────────────────┘         └─────────────────────┘
+              │                               │
+              │   If timer fires first:       │
+              │   ◀─────────────────────────────
+              │         sendEmail()           │
+              │                               │
+              ▼                               │
+   ┌─────────────────────┐                    │
+   │ Wait for processing │◀───────────────────┘
+   │ to complete         │
+   └─────────────────────┘
+```
+
+**Use case:** Order processing with SLA monitoring - notify customer if processing is delayed, but don't cancel the operation.
+
+## Prerequisites
+
+1. Cadence server running (see [main README](../../../../README.md))
+2. Build the samples: `make`
+
+## Running the Sample
+
+```bash
+# Terminal 1: Start worker
+./bin/timer -m worker
+
+# Terminal 2: Trigger workflow
+./bin/timer -m trigger
+```
+
+**Possible outcomes:**
+- Processing finishes in < 3 seconds → Timer cancelled, no email
+- Processing takes > 3 seconds → Email sent, then wait for completion
+
+## Key Code
+
+```go
+// Start long-running activity
+f := workflow.ExecuteActivity(ctx, orderProcessingActivity)
+selector.AddFuture(f, func(f workflow.Future) {
+    processingDone = true
+    cancelHandler()  // Cancel timer if processing finishes first
+})
+
+// Start timer for notification threshold
+timerFuture := workflow.NewTimer(childCtx, processingTimeThreshold)
+selector.AddFuture(timerFuture, func(f workflow.Future) {
+    if !processingDone {
+        workflow.ExecuteActivity(ctx, sendEmailActivity)  // Send notification
+    }
+})
+
+selector.Select(ctx)  // Wait for either to complete
+```
+
+## Testing
+
+```bash
+go test -v ./cmd/samples/recipes/timer/
+```
+
+## References
+
+- [Cadence Timers](https://cadenceworkflow.io/docs/go-client/timers/)
+- [Workflow Selectors](https://cadenceworkflow.io/docs/go-client/selectors/)
+