Add README for cmd/samples/recipes/childworkflow

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

Author: zawadzkidiana

cmd/samples/recipes/childworkflow/README.md

@@ -0,0 +1,89 @@
+# Child Workflow Sample
+
+This sample demonstrates **parent-child workflow relationships** and the **ContinueAsNew** pattern for long-running workflows.
+
+> **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
+
+```
+┌─────────────────────────────────────────────────────┐
+│                 Parent Workflow                      │
+│  ExecuteChildWorkflow(child, 0, 5)                  │
+│                      │                               │
+│                      ▼                               │
+│  ┌─────────────────────────────────────────────┐    │
+│  │           Child Workflow                     │    │
+│  │  Run 1 → ContinueAsNew                       │    │
+│  │  Run 2 → ContinueAsNew                       │    │
+│  │  Run 3 → ContinueAsNew                       │    │
+│  │  Run 4 → ContinueAsNew                       │    │
+│  │  Run 5 → Complete ✓                          │    │
+│  └─────────────────────────────────────────────┘    │
+│                      │                               │
+│                      ▼                               │
+│  Parent receives: "completed after 5 runs"          │
+└─────────────────────────────────────────────────────┘
+```
+
+**Use cases:**
+- Breaking large workflows into modular pieces
+- Long-running workflows that need to reset history (ContinueAsNew)
+- Workflow decomposition for better organization
+
+## Prerequisites
+
+1. Cadence server running (see [main README](../../../../README.md))
+2. Build the samples: `make`
+
+## Running the Sample
+
+```bash
+# Terminal 1: Start worker
+./bin/childworkflow -m worker
+
+# Terminal 2: Trigger workflow
+./bin/childworkflow -m trigger
+```
+
+## Key Code
+
+### Parent Workflow
+```go
+cwo := workflow.ChildWorkflowOptions{
+    WorkflowID:                   childID,
+    ExecutionStartToCloseTimeout: time.Minute,
+}
+ctx = workflow.WithChildOptions(ctx, cwo)
+
+var result string
+err := workflow.ExecuteChildWorkflow(ctx, sampleChildWorkflow, 0, 5).Get(ctx, &result)
+```
+
+### Child Workflow with ContinueAsNew
+```go
+func sampleChildWorkflow(ctx workflow.Context, totalCount, runCount int) (string, error) {
+    totalCount++
+    runCount--
+    
+    if runCount == 0 {
+        return fmt.Sprintf("completed after %v runs", totalCount), nil
+    }
+    
+    // Restart workflow with new parameters (resets history)
+    return "", workflow.NewContinueAsNewError(ctx, sampleChildWorkflow, totalCount, runCount)
+}
+```
+
+## Why ContinueAsNew?
+
+Workflow history grows with each event. For long-running workflows, use `ContinueAsNew` to:
+- Reset the history size
+- Prevent hitting history limits
+- Keep workflows performant
+
+## References
+
+- [Child Workflows](https://cadenceworkflow.io/docs/go-client/child-workflows/)
+- [ContinueAsNew](https://cadenceworkflow.io/docs/go-client/continue-as-new/)
+