Hi,

I noticed the image_url field doesn't seem to be listed in the docs. It might be useful to add it and also describe how the profile fields are used.

Suggested diff below.

Thanks, Aonrud

diff --git a/docs/user_guide.md b/docs/user_guide.md
index 6df4859..3e10057 100644
--- a/docs/user_guide.md
+++ b/docs/user_guide.md
@@ -25,9 +25,10 @@ As these two config items define your ActivityPub handle `@handle@domain`.
 
 You can tweak your profile by tweaking these items:
 
- - `name`
- - `summary` (using Markdown)
- - `icon_url`
+ - `name`: The name shown with your profile.
+ - `summary`: The summary or 'bio' part of your profile, written in Markdown.
+ - `icon_url`: Your profile image or avatar.
+ - `image_url`: This provides a 'header' or 'banner' image. Note that it is not shown by the default Microblog.pub templates. It will be used by Mastodon (which uses a 3:1 ratio image) and Pleroma. Pixelfed and Peertube, for example, don't show these images by default.
 
 Whenever one of these config items is updated, an `Update` activity will be sent to all known servers to update your remote profile.