Create blog posts directory with initial post
Create blog posts directory with initial post

This sets up the `_posts` folder where all blog posts should be published.
There's a first blog post example that is the verbatim post @yeastplume did
to announce the release of v4.0.0 on the forums.

Important! Each blog post must have "front matter" where:
* layout is set to `post`
* title is given to the blog post
* author is specified
* and categories is set to `blog`, in order to ensure the post is published under `/blog` on the website.

mimblewimble/site,mimblewimble/site,mimblewimble/site