Add .gitignore best practices

[chuckwondo]

To help reduce the chances of users committing files that should not be committed, add a section regarding .gitignore best practices (or perhaps more generally, Git Best Practices), where we recommend that when creating a new repo, a .gitignore files should be initialized (or even extended in existing repos) by using the following commands to add recommended patterns to ignore:

curl -sL https://www.toptal.com/developers/gitignore/api/r >> .gitignore
curl -sL https://www.toptal.com/developers/gitignore/api/python >> .gitignore
curl -sL https://www.toptal.com/developers/gitignore/api/jupyternotebooks >> .gitignore
curl -sL https://www.toptal.com/developers/gitignore/api/linux >> .gitignore
curl -sL https://www.toptal.com/developers/gitignore/api/macos >> .gitignore
curl -sL https://www.toptal.com/developers/gitignore/api/windows >> .gitignore
curl -sL https://www.toptal.com/developers/gitignore/api/vim >> .gitignore