Add info to readme for writing blog posts

Signed-off-by: David Martin <[email protected]>

README.md

@@ -17,3 +17,30 @@ You will need:
 
 # Publishing
 Published via GH action, see `.github/workflows/eleventy_build.yml`
+
+# Blog
+
+Blog posts are stored as markdown files in `src/blog/`.
+The name of the file doesn't matter.
+Metadata is included at the top of the file like this:
+
+```markdown
+---
+title: My blog post
+date: 2024-08-02
+author: Me
+---
+```
+
+This will ensure the blog post title shows up in the post list, grouped by year and sorted by date.
+The title, author and date will also render in a styled manner at the top of the post page.
+
+If you want to include images, you can create a folder in the `src/blog/` folder, and put the markdown file there, along with any images.
+Then you can insert an image like this:
+
+```njk
+{% image "./myimage.png", "My image" %}
+```
+
+The post list template is at `src/_includes/postslist.njk`.
+The post page template is at `src/_includes/layouts/post.njk`.