Summary

Create a README for the _posts directory. Use the previous posts README as a starting point.

Details

This issue originated from this comment by @mejiaj in #2979:

issue: If we're reviving this section, we should bring back and update the README. With this guidance anyone could create a post.

https://github.com/uswds/uswds-site/blob/01743d4e511b7cf83788d0cdeecafbecd1da9ada/_posts/README.md