Incluse contributor policies

[antaldaniel]

Editor comments:

• The contributing guide does not seem customized. It mentions a possible "src" folder which is not present.
• You might want to rename the master branch to main as some people can be offended by the word "master", see https://www.tidyverse.org/blog/2021/10/renaming-default-branch/ that includes links to context, and practical advice on renaming the default branch.
• The iris dataset is very well-known, but it is also infamous because of its eugenics links.
  Since having a good example dataset is very important, would you consider replacing it with another one, like maybe the palmerpenguins one, even if it comes at the cost of adding a (possibly optional) dependency?