Create contributing guide

Before I read the article about skeleton, I was confused about various files such as *.egg-info etc. not being automatically excluded by .gitignore in one of the downstream skeleton projects, because repo-local .gitignore files are a very common practice nowadays.

Only later I found this https://blog.jaraco.com/skeleton/#ignoring-artifacts.

Making a simple guide for new contributors might be helpful to get them started and prevent from commiting files they don't intend to commit.

Start by reading the linked skeleton article, especially its section on ignoring artifacts, and inspect the repository for existing contributor documentation. Add a simple guide for new contributors that explains files such as *.egg-info and how to avoid committing unintended artifacts; done means the guidance is easy to find and addresses the confusion described in the issue.

Written by the indexing model from the issue text.