update 04-blackcat readme with screenshot

.gitignore

@@ -9,6 +9,7 @@
 *.user
 *.sln.docstates
 *.userprefs
+*.DS_Store
 
 # Xamarin.Android
 Resource.designer.cs

Chapter04/FS/BlackCat/README.md

@@ -12,14 +12,23 @@ urlFragment: chapter04-fs-blackcat
 
 Simple Xamarin.Forms application written in F#, that loads text from a file, to show on the screen:
 
+![iOS app showing text loaded from file](Screenshots/01.png)
+
 ```fsharp
-    // Get access to the text resource.
-    let assembly = base.GetType().GetTypeInfo().Assembly
-    let resource = "TheBlackCat.txt"
+// Get access to the text resource.
+let assembly = base.GetType().GetTypeInfo().Assembly
+let resource = "TheBlackCat.txt"
 
-    // Get all the lines of the file.
-    let lines = seq { use stream = assembly.GetManifestResourceStream(resource)
-                      use reader = new StreamReader(stream)
-                      while not reader.EndOfStream do
-                          yield reader.ReadLine() }
+// Get all the lines of the file.
+let lines = seq { use stream = assembly.GetManifestResourceStream(resource)
+                    use reader = new StreamReader(stream)
+                    while not reader.EndOfStream do
+                        yield reader.ReadLine() }
 ```
+
+> [!NOTE]
+> **BlackCatPage.fs** line 21 might need to be updated to:
+>
+> `let resource = "BlackCat.TheBlackCat.txt"`
+>
+> If you experience a null reference exception.

Chapter04/FS/ColorBlocks/README.md

@@ -8,7 +8,7 @@ products:
 - xamarin
 urlFragment: chapter04-fs-colorblocks
 ---
-# Greetings F\#
+# Greetings F\# in Xamarin.Forms
 
 Simple Xamarin.Forms application written in F#, rendering blocks of color: