Add information about access modifiers (#24422)

* Add information about access modifiers

Added some information about 3 most common access modifiers and a simple example.

* Fix indent

guide/english/csharp/access-modifiers/index.md

@@ -0,0 +1,28 @@
+---
+title: Access modifiers
+---
+
+# Access modifiers
+
+Access modifiers are keywords used to specify the declared accessibility of a member or a type. 
+
+Here are the 3 most common access modifiers:
+* `public` - access is not restricted.
+* `protected` - access is limited to the containing class or types derived from the containing class.
+* `private` - access is limited to the containing type.
+
+## Examples
+```csharp
+public class Person
+{
+  public string FirstName { get; set;} // these can be accessed from anywhere
+  public string LastName { get; set;}
+    
+  protected string SecretMessage { get; set;} // this can be accessed from this class or any derived classes
+    
+  private int PersonId { get; set;} // this can only be accessed from this class
+}
+```
+
+## Sources
+- [Microsoft](https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/access-modifiers)