Add Contributing.md

[marinapopovic11]

Let's add some contributing file, so we can work together, better!!!

[mannau]

Dear Marina,

thanks for your contribution! For a successful pull request please

1. Make sure that all checks/test cases run through
2. Improve the documentation (e.g. for meanimputation)
3. Review the function windsorize for the vector c(1, NA)

Cheers,
m