Added nPr Code in C# and a README file to understand it fully.

[subhadeep1912]

Description

I have done the nPr problem in C#.

Reference Issue number #330 issue_no must refer to the issue related to this PR, visit here

Type of change

Choosing one or more options from the following as per the nature of your Pull request. NOTE: Check boxes using [x]

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [x] Documentation Update

Checklist:

• [x] I have named my files and folder, according to this project's guidelines.
• [x] My code follows the style guidelines of this project.
• [x] My Pull Request has a descriptive title. (not a vague title like Update index.md)
• [ ] I have commented on my code, particularly in hard-to-understand areas.
• [x] I have created a helpful and easy to understand README.md, with problem description and my name.
• [ ] I have included a requirements.txt file (if external libraries are required.)
• [x] My changes do not produce any warnings.
• [x] I have starred this repository.
• [ ] I have added a working sample/screenshot of the script.
• [x] I have checked for trailing spaces in file names and none have them.