Added CONTRIBUTING.md file

[amitkumar-y]

As mentioned in https://github.com/FireCubeStudios/FluentFlyouts3/issues/13 I have added the standard details in the CONTRIBUTING.md file, let me know if you want any specific details that needs be the part of the file, I will make the changes accordingly.

Once we are done with CONTRIBUTING.md file we can also add CODE_OF_CONDUCT.md file.

[FireCubeStudios]

I think we should add a section on how to create your own flyout. the problem is I havent implemented that yet so it will have to wait for a bit. Can we create code of conduct now?

[amitkumar-y]

Sure will do that.

[FireCubeStudios]

I will update this when I get the time