Better surface diff tool configuration (pointfreeco#469)

* Update README.md

* Update AssertSnapshot.swift

* Update AssertSnapshot.swift

Author: stephencelis

README.md

@@ -195,7 +195,10 @@ end
   - **Supports any platform that supports Swift.** Write snapshot tests for iOS, Linux, macOS, and tvOS.
   - **SceneKit, SpriteKit, and WebKit support.** Most snapshot testing libraries don't support these view subclasses.
   - **`Codable` support**. Snapshot encodable data structures into their [JSON](Documentation/Available-Snapshot-Strategies.md#json) and [property list](Documentation/Available-Snapshot-Strategies.md#plist) representations.
-  - **Custom diff tool integration**.
+  - **Custom diff tool integration**. Configure failure messages to print diff commands for [Kaleidoscope](https://kaleidoscope.app) (or your diff tool of choice).
+    ``` swift
+    SnapshotTesting.diffTool = "ksdiff"
+    ```
 
 ## Plug-ins
 

Sources/SnapshotTesting/AssertSnapshot.swift

@@ -288,7 +288,16 @@ public func verifySnapshot<Value, Format>(
 
       let diffMessage = diffTool
         .map { "\($0) \"\(snapshotFileUrl.path)\" \"\(failedSnapshotFileUrl.path)\"" }
-        ?? "@\(minus)\n\"\(snapshotFileUrl.path)\"\n@\(plus)\n\"\(failedSnapshotFileUrl.path)\""
+        ?? """
+        @\(minus)
+        "\(snapshotFileUrl.path)"
+        @\(plus)
+        "\(failedSnapshotFileUrl.path)"
+
+        To configure output for a custom diff tool, like Kaleidoscope:
+
+            SnapshotTesting.diffTool = "ksdiff"
+        """
       return """
       Snapshot does not match reference.