Update the README with some of the new features

go/blog/README.md

- post types (see [blog.yaml](./blog.yaml) for usage examples)
    - `shell`: Write about an interesting shell command
    - `link`: Quickly post a link
    - `image`: Post an image
    - `song`: Post a song (using `<audio>`)
    - `text`: Write an actual blog post (or just a heading)
    - `video`: Post a video (from YouTube)
- a simple tagging system