BakeRenderTexture.cs

using UnityEngine;
using UnityEditor;
using System.IO;

public class BakeRenderTexture
{
    [MenuItem("Assets/Bake RenderTexture to PNG")]
    static void Bake()
    {
        // Get selected RenderTexture
        RenderTexture rt = Selection.activeObject as RenderTexture;
        if (!rt) return;

        // Backup current state
        RenderTexture backup = RenderTexture.active;

        // Allocate CPU memory
        var texture = new Texture2D(rt.width, rt.height, TextureFormat.ARGB32, false);

        // Read from GPU memory to CPU memory
        RenderTexture.active = rt;
        texture.ReadPixels(new Rect(0, 0, rt.width, rt.height), 0, 0);

        // Save
        var path = AssetDatabase.GenerateUniqueAssetPath(Path.ChangeExtension(AssetDatabase.GetAssetPath(rt), "png"));
        File.WriteAllBytes(path, texture.EncodeToPNG());

        // Free CPU memory
        Object.DestroyImmediate(texture);

        // Restore state
        RenderTexture.active = backup;

        // Refresh
        AssetDatabase.Refresh();
    }
}