Small Note on Simulator/OS Discrepancies (#162)

* Update README.md

* Update Available-Snapshot-Strategies.md

* Update README.md

Author: stephencelis

Documentation/Available-Snapshot-Strategies.md

@@ -259,6 +259,8 @@ assertSnapshot(matching: image, as: .image(precision: 0.99)
 
 A snapshot strategy for comparing layers based on pixel equality.
 
+> Note: Snapshots must be compared on the same OS as the device that originally took the reference to avoid discrepancies between images.
+
 **Format:** `NSImage`
 
 **Note:** Includes `SCNView`, `SKView`, `WKWebView`.
@@ -322,6 +324,8 @@ A=autoresizesSubviews, C=canDrawConcurrently, D=needsDisplay, F=flipped, G=gstat
 
 ## NSViewController
 
+> Note: Snapshots must be compared on the same OS as the device that originally took the reference to avoid discrepancies between images.
+
 ### `.image`
 
 A snapshot strategy for comparing layers based on pixel equality.
@@ -485,6 +489,8 @@ assertSnapshot(matching: image, as: .image(precision: 0.99)
 
 A snapshot strategy for comparing layers based on pixel equality.
 
+> Note: Snapshots must be compared using a simulator with the same OS, device gamut, and scale as the simulator that originally took the reference to avoid discrepancies between images.
+
 **Format:** `UIImage`
 
 **Note:** Includes `SCNView`, `SKView`, `WKWebView`.
@@ -602,6 +608,8 @@ Records:
 
 A snapshot strategy for comparing layers based on pixel equality.
 
+> Note: Snapshots must be compared using a simulator with the same OS, device gamut, and scale as the simulator that originally took the reference to avoid discrepancies between images.
+
 **Format:** `UIImage`
 
 #### Parameters:

README.md

@@ -73,6 +73,8 @@ assertSnapshot(matching: vc, as: .image(on: .iPadMini(.portrait)))
 assertSnapshot(matching: vc, as: .recursiveDescription(on: .iPadMini(.portrait)))
 ```
 
+> Note: Snapshots must be compared using a simulator with the same OS, device gamut, and scale as the simulator that originally took the reference to avoid discrepancies between images.
+
 Better yet, SnapshotTesting isn't limited to views and view controllers! There are [a number of available snapshot strategies](Documentation/Available-Snapshot-Strategies.md) to choose from.
 
 For example, you can snapshot test URL requests (_e.g._, those that your API client prepares).