Add README.md for DotNetSolutionsMerger tool

This commit introduces the README.md file for the DotNetSolutionsMerger tool. The README includes sections on the tool's overview, installation instructions, command line usage, and examples. It provides essential details and instructions intended to help users understand how to use the tool.

README.md

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+## 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.
+
+## Installation
+Ensure you have .NET SDK installed. Clone or download the repository containing this tool, then build it using the .NET CLI.
+
+Download the repository and navigate to the project directory and run the following command:
+
+```bash
+dotnet build DotNetSolutionsMerger.sln
+```
+
+Or just download the latest release (dll) from the [Releases](https://github.com/rodion-m/DotNetSolutionsMerger/releases)
+
+## Usage
+The tool can be invoked using the command line. Below are the usage options and examples.
+
+### Syntax
+```bash
+dotnet MergeSolutions.dll --input <input_paths> --output <output_path>
+```
+
+### Options
+- `--input`, `-i`: Input solution file paths or a directory containing solution files. This option is required and can accept multiple paths.
+- `--output`, `-o`: Output path for the merged solution file. This option is required.
+
+### Examples
+
+#### Example 1: Merging multiple solution files
+To merge several solution files into a single solution file:
+```bash
+dotnet MergeSolutions.dll --input solution1.sln solution2.sln solution3.sln --output mergedSolution.sln
+```
+This command merges `solution1.sln`, `solution2.sln`, and `solution3.sln` into `mergedSolution.sln`.
+
+#### Example 2: Merging all solution files in a directory
+To merge all solution files located in a specific directory:
+```bash
+dotnet MergeSolutions.dll --input /path/to/solutions --output mergedSolution.sln
+```
+This command merges all solution files found in `/path/to/solutions` directory into `mergedSolution.sln`.