Add QuickSort [C#]

[sebasrp]

Fixes #493

By submitting this pull request I confirm I've read and complied with the below declarations.

• I have read the Contribution guidelines and I am confident that my PR reflects them.
• I have followed the coding guidelines for this project.
• My code follows the skeleton code structure.
• This pull request has a descriptive title. For example, Added {Algorithm/DS name} [{Language}], not Update README.md or Added new code.
• This pull request will be closed if I fail to update it even once in a continuous time span of 7 days.

[singhpratyush]

Hi! Thanks for showing interest in contributing to this repository.

Please make sure that you have ticked the points mentioned in PR description correctly.

Thanks again!

[sebasrp]

I am not sure why gitmate failed and how to restart the check. Can someone let me know how to do that?
Thanks

[DimaMukhin]

Looks like a pull request to an issue I was assigned to (#493)
Thank you for the help, it looks very good, but I feel like we can cover way more algorithms by working on separate issues. I have already started working on QuickSort

[singhpratyush]

@sebasrp: Thanks for submitting the code. But as @DimaMukhin was assigned to the issue, he is the one who sends a pull request. Please read contributing guidelines carefully. :)

[sebasrp]

@DimaMukhin and @singhpratyush
I understand - kind of sad we are throwing away a good piece of work.
I had read the contribution guidelines, what confused me (and I think you should reconsider fixing those) is that the issue was labeled as Hacktoberfest. This is a month long event that encourages people to look for open issues in repos they wouldn't contribute normally to help out with some Pull Request.
I would recommend removing the tag if the issue is already assigned and being worked on in order to avoid these occurrences where someone with good intentions submit something that will not be considered.