Flagr .net client added to readme

[UmutComlekci]

Description

Flagr .Net Client published to nuget and Flagr readme file updated for inform everyone

Motivation and Context

Use Flagr with many other language include .Net

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)

Checklist:

• [X] My code follows the code style of this project.
• [X] My change requires a change to the documentation.
• [X] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [X] All new and existing tests passed.