Print total size of compressed layers (#60)

Before pulling an image.

README.md

@@ -160,3 +160,23 @@ tart pull acme.io/remoteorg/name:latest my-local-vm-name
   Tart is a relatively small project, and it didn't feel right to try to monetize it.
   Apple did all the heavy lifting with their `Virtualization.Framework`.
 </details>
+
+<details>
+  <summary>How to change VM's disk size?</summary>
+
+  You can choose disk size upon creation of a virtual machine:
+
+  ```shell
+  tart create --from-ipsw=latest --disk-size=25 monterey-vanilla
+  ```
+
+  For an existing VM please use [Packer Plugin](https://github.com/cirruslabs/packer-plugin-tart) which can increase
+  disk size for new virtual machines. Here is an example of [how to change disk size in a Packer template](https://github.com/cirruslabs/macos-image-templates/blob/fb0bcf68e0b093129136875c050205a66729b596/templates/base.pkr.hcl#L15).
+</details>
+
+<details>
+  <summary>Nested virtualization support?</summary>
+
+  Tart is limited by functionality of Apple's `Virtualization.Framework`. At the moment `Virtualization.Framework`
+  doesn't support nested virtualization.
+</details>

Sources/tart/VMDirectory+OCI.swift

@@ -52,8 +52,10 @@ extension VMDirectory {
     }
 
     // Progress
-    defaultLogger.appendNewLine("pulling disk...")
-    let progress = Progress(totalUnitCount: Int64(diskLayers.map{ $0.size }.reduce(0) { $0 + $1 }))
+    let diskCompressedSize: Int64 = Int64(diskLayers.map {$0.size}.reduce(0) {$0 + $1})
+    let prettyDiskSize = String(format: "%.1f", Double(diskCompressedSize) / 1_000_000_000.0)
+    defaultLogger.appendNewLine("pulling disk (\(prettyDiskSize) GB compressed)...")
+    let progress = Progress(totalUnitCount: diskCompressedSize)
     ProgressObserver(progress).log(defaultLogger)
 
     for diskLayer in diskLayers {