Add authorship note in README.md

Inserted a sentence in README.md declaring Claude Sonnet 3.5 as the major contributor of the code. The added information serves to credit the author and indicate the person responsible for a lot of the source code.

README.md

@@ -1,6 +1,8 @@
 ## Overview
 This tool merges multiple .NET solution files into a single solution file. It is designed to be run from the command line and accepts various input options for flexibility.
 
+_The source code is almost entirely written by Claude Sonnet 3.5._
+
 ## Installation
 Ensure you have .NET SDK installed. Clone or download the repository containing this tool, then build it using the .NET CLI.