readme: document user templates (#99)

* document user_templates in README

* move user_template documentation to Usage

* format

---------

Co-authored-by: 2 * r + 2 * t <61896496+soramanew@users.noreply.github.com>

README.md

@@ -26,11 +26,13 @@ The main control script for the Caelestia dotfiles.
 > For automatic Papirus folder icon color syncing, `papirus-folders` needs to be able to run with `sudo` without a password prompt.
 >
 > **Recommended** - Create a sudoers file:
+>
 > ```fish
 > # Fish shell
 > echo "$USER ALL=(ALL) NOPASSWD: "(which papirus-folders) | sudo tee /etc/sudoers.d/papirus-folders
 > sudo chmod 440 /etc/sudoers.d/papirus-folders
 > ```
+>
 > ```sh
 > # Bash/other shells
 > echo "$USER ALL=(ALL) NOPASSWD: $(which papirus-folders)" | sudo tee /etc/sudoers.d/papirus-folders
@@ -38,6 +40,7 @@ The main control script for the Caelestia dotfiles.
 > ```
 >
 > **Alternatively** - Edit the main sudoers file by running `sudo visudo` and adding at the end:
+>
 > ```
 > your_username ALL=(ALL) NOPASSWD: /usr/bin/papirus-folders
 > ```
@@ -148,6 +151,24 @@ subcommands:
     resizer      window resizer daemon
 ```
 
+### User Templates
+
+Custom user templates can be defined in `~/.config/caelestia/templates/`.
+
+#### Template syntax
+
+`{{ <color>.<format> }}`
+
+- `<color>` is a theme color role derived from the Material You color system (e.g. `primary`, `secondary`, `background`)
+- `<format>` is the output format: `hex` or `rgb`
+
+#### Examples
+
+- `{{ primary.hex }}` outputs `3f4ba2`
+- `{{ primary.rgb }}` outputs `rgb(193, 132, 207)`
+
+Output files are written to `~/.local/state/caelestia/theme/`. You can symlink them to your desired locations.
+
 ## Configuring
 
 All configuration options are in `~/.config/caelestia/cli.json`.