Add README_RUN.md to pkg/app for better understanding Run struct

Signed-off-by: yxxhero <aiopsclub@163.com>

pkg/app/README_RUN.md

@@ -0,0 +1,72 @@
+# Run Struct
+
+## Overview
+
+The `Run` struct is a core component of Helmfile's execution model, responsible for orchestrating the preparation, execution, and cleanup phases of Helmfile commands. It provides a consistent workflow for all Helmfile operations while maintaining separation of concerns.
+
+## Core Mechanics
+
+### The Preparation-Execution-Cleanup Pattern
+
+At the heart of the `Run` package is the `withPreparedCharts` method, which implements a functional programming pattern to standardize how Helmfile commands are executed:
+
+```go
+func (r *Run) withPreparedCharts(helmfileCommand string, opts state.ChartPrepareOptions, f func()) error {
+    // Preparation phase
+    // ...
+
+    // Execution phase
+    f()
+
+    // Cleanup phase
+    // ...
+}
+```
+
+This pattern consists of three distinct phases:
+
+1. **Preparation Phase**:
+   - Validates the current state
+   - Syncs repositories if needed
+   - Creates temporary directories for chart downloads
+   - Triggers global prepare events
+   - Downloads and processes charts
+
+2. **Execution Phase**:
+   - Executes the command-specific logic provided as a function parameter
+   - Command implementations are decoupled from chart preparation
+
+3. **Cleanup Phase**:
+   - Triggers global cleanup events
+   - Handles resource cleanup
+
+### Usage Example
+
+Commands like `diff`, `apply`, or `template` use this pattern by providing their specific implementation as a function:
+
+```go
+// Example: How the diff command uses withPreparedCharts
+err := run.withPreparedCharts("diff", opts, func() {
+    msg, matched, affected, errs = a.diff(run, c)
+})
+```
+
+## Benefits
+
+- **Consistent Workflow**: All commands follow the same preparation, execution, and cleanup flow
+- **Separation of Concerns**: Chart preparation logic is centralized and reused across commands
+- **Resource Management**: Temporary resources are properly created and cleaned up
+- **Extensibility**: New commands can be added by implementing their specific logic without duplicating preparation code
+
+## Key Components
+
+- **ReleaseToChart**: Maps release identifiers to their prepared chart paths
+- **Chart Preparation**: Handles downloading, modifying, and preparing charts for use
+- **Repository Synchronization**: Ensures all required Helm repositories are available
+- **Event Triggers**: Executes global prepare and cleanup events at appropriate times
+
+## Implementation Details
+
+The `Run` struct maintains state throughout the execution lifecycle, ensuring that charts are prepared only once and that all commands have access to the prepared charts through the `ReleaseToChart` map.
+
+Commands can focus on their specific implementation while relying on the `withPreparedCharts` method to handle all the common preparation and cleanup tasks.