Update README and document theme variables

Author: matze

README.md

@@ -1,18 +1,21 @@
-## splat
+# 📷 splat
 
 ![Rust](https://github.com/matze/splat/workflows/Rust/badge.svg)
 
-A simple [sigal](https://github.com/saimn/sigal) clone written in Rust.
+**splat** is a command line application to generate static photo galleries from
+a source directory of image files. It is a spiritual cousin of
+[sigal](https://github.com/saimn/sigal) but written in Rust.
 
 <a href="https://matze.github.io/splat/"><img alt="Example output" src="https://github.com/matze/splat/blob/master/example/screenshot.jpg"/></a>
 
 <p align="center"><strong><a href="https://matze.github.io/splat/">DEMO OUTPUT</a></strong></p>
 
-### Usage
 
-splat is a command line application to generate static photo galleries from a
-source directory of image files, a `splat.toml` configuration file and a theme
-file containing an HTML template and optional assets.
+## Usage
+
+**splat** is a command line application and besides a source directory of image
+files it requires a `splat.toml` configuration file and a theme file containing
+an HTML template as well as optional assets.
 
 To create an example configuration run `splat new` and edit `splat.toml` to your
 liking, especially adapt the `input` and `output` paths. `output` will be
@@ -22,3 +25,29 @@ output.
 > [!IMPORTANT]
 > The example theme relies on the Tailwind CSS v4.0 beta compiler. Make sure to
 > install it if you want to use the theme.
+
+
+## Templates
+
+Templates must be written in [tera
+syntax](https://keats.github.io/tera/docs/#templates). The following hierarchy
+of variables is available:
+
+- `collection`
+  - `title` of this collection
+  - `breadcrumbs`
+    - `path` to the corresponding page
+    - `title` of the corresponding page
+- `children` of sub-collections
+  - `path` to the collection
+  - `title` of the collection
+  - `thumbnail` of the collection
+- `images` for this collection
+  - `path` to the image
+  - `width` and `height` of the image
+  - `thumbnail` of the image
+
+
+## License
+
+[MIT](./LICENSE)