Updated readme and improved running instance behavior

LunaWin/LunaWin/Program.cs

@@ -47,8 +47,9 @@ namespace LunaWin
                 }
             } else
             {
+                MessageBox.Show("LunaWin is already running.");
                 Application.Exit();
             }
         }
     }
-}
+}

README.md

@@ -14,6 +14,7 @@ This project demonstrates a number of concepts, some more advanced than others.
 
 **Advanced**
 
-* Using [`DllImport`](https://learn.microsoft.com/en-us/dotnet/api/system.runtime.interopservices.dllimportattribute?view=net-8.0) to access the current Windows 10+ theme (light or dark) natively
+* Using [`Mutex`](https://learn.microsoft.com/en-us/dotnet/api/system.threading.mutex?view=netframework-4.7.2) to find exisiting instance of application
+* Using [`DllImport`](https://learn.microsoft.com/en-us/dotnet/api/system.runtime.interopservices.dllimportattribute?view=netframework-4.7.2) to access the current Windows 10+ theme (light or dark) natively
 * Overloading [`WndProc`](https://learn.microsoft.com/en-us/dotnet/api/system.windows.forms.control.wndproc?view=windowsdesktop-8.0) to handle system-wide messaging, to detect Windows 10+ theme changes
 * Accessing memory directly through pointers and [`unsafe`](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/unsafe) code handling