Write docs for src/Files.App/Extensions

Author: Lamparter

src/Files.App/Extensions/README.md

@@ -0,0 +1,23 @@
+# `src/Files.App/Extensions`
+
+This folder contains extension classes that provide methods for performing operations such as localization and string manipulation.
+
+---
+
+For example, it contains the source file `Fractions.cs` which allows for converting double values into fractions.
+It also contains `LocalizationExtensions.cs` which allows the developer to insert a string with the following format:
+
+```cs
+using Files.App.Extensions;
+
+namespace Files.Example;
+
+class Program
+{
+    static void Main(string[] args)
+    {
+        private string _exampleResource
+            => "ExampleStringID".ToLocalized();
+    }
+}
+```