Add package installation to README

[timheuer]

Add package installation snippet
In readme, just following standard NuGet readme help with the installation snippet

Motivation and Context

Additionally getting started info

How Has This Been Tested?

doc only

Breaking Changes

No, doc only

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context