Add documentation about image_url and custom favicon

Also expand documentation about custom templates

docs/user_guide.md

@@ -28,6 +28,7 @@ You can tweak your profile by tweaking these items:
  - `name`
  - `summary` (using Markdown)
  - `icon_url`
+ - `image_url`
 
 Whenever one of these config items is updated, an `Update` activity will be sent to all known servers to update your remote profile.
 
@@ -35,6 +36,15 @@ The server will need to be restarted for taking changes into account.
 
 Before restarting the server, you can ensure you haven't made any mistakes by running the [configuration checking task](/user_guide.html#configuration-checking).
 
+Note that currently `image_url` is not used anywhere in microblog.pub itself, but other clients/servers do occasionally use it when showing remote profiles as a background image.
+Also, this image _can_ be used in microblog.pub - just add this:
+
+```html
+<img src="{{ local_actor.image_url | media_proxy_url }}">
+```
+
+to an appropriate place of your template (most likely, `header.html`).
+For more information, see a section about [custom templates](/user_guide.html#custom-templates) further in this document.
 
 ### Profile metadata
 
@@ -161,10 +171,35 @@ $secondary-color: #32cd32;
 
 See `app/scss/main.scss` to see what variables can be overridden.
 
+You will need to [recompile CSS](#recompiling-css-files) after doing any CSS changes (for actual css files to be updates) and restart microblog.pub (for css link in HTML documents to be updated with a new checksum - otherwise, browsers that downloaded old CSS will keep using it).
+
+#### Custom favicon
+
+By default, microblog.pub favicon is a square of `$primary-color` CSS color (see above section on how to redefine CSS colors).
+You can change it to any icon you like - just save a desired file as `data/favicon.ico`.
+After that, run the "[recompile CSS](#recompiling-css-files)" task to copy it to `app/static/favicon.ico`.
+
 #### Custom templates
 
 If you'd like to customize your instance's theme beyond CSS, you can modify the app's HTML by placing templates in `data/templates` which overwrite the defaults in `app/templates`.
 
+Templates are written using [Jinja](https://jinja.palletsprojects.com/en/latest/templates/) templating language.
+Moreover, `utils.html` has scoped blocks around the body of every macro.
+This allows macros to be overridden individually in `data/templates/utils.html`, without copying the whole file.
+For example, to only override the display of a specific actor's name/icon, you can create `data/templates/utils.html` file with following content:
+
+```jinja
+{% extends "app/utils.html" %}
+
+{% block display_actor %}
+	{% if actor.ap_id == "https://me.example.com" %}
+		<!-- custom actor display -->
+	{% else %}
+		{{ super() }}
+	{% endif %}
+{% endblock %}
+```
+
 #### Custom Content Security Policy (CSP)
 
 You can override the default Content Security Policy by adding a line in `data/profile.toml`: