Add "Make a blog" howto

[spanezz]

Add an HOWTO on how to add a blog to a site

[spanezz]

Now added as a quickstart in 13e0133659089b85bedf6fd512f501d93cafd3d0.

It's very basic, and there should be a few more blog-related howtos:

• [x] add an about page
• [x] collect posts into a posts/ directory
• [x] collect posts into a directory per year
• [x] add tags to posts
• [x] serialize posts in series

[spanezz]

There should be enough now to get one started with some nontrivial blogging