Consider blog & case studies contribution guidelines

Now that we have [community members contributing blog articles](https://github.com/etcd-io/website/pulls?q=is%3Aopen+is%3Apr+label%3Ablog) (thanks! this is awesome), we should consider setting guidelines for those submissions.

Kubernetes has a page describing "[submitting blog posts and case studies](https://kubernetes.io/docs/contribute/new-content/blogs-case-studies/)" that we could use as a sample/starting point (though I'd expect what we are looking for may be different).

Do we want blog contribution guidelines? If so, what form should they take?