Clarify contribution guidelines for inside Rust blog

[yoshuawuyts]

As per #1011 (comment) the procedure to contribute to the inside Rust blog currently appears to be undocumented. It'd be good to clarify for contributors what the purpose of the blog is, who should review the post, and when / by whom the posts can be merged.

I think ideally this should be documented in a CONTRIBUTING.md file which is usually a good way to describe policies like those, and integrates directly into the GitHub UI as well.