doc(readme): add description and examples

.gitignore

@@ -1,6 +1,6 @@
 out
 dist
-*.png
+/*.png
 *.keymap
 *.json
 coverage.txt

README.md

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 ![GitHub go.mod Go version](https://img.shields.io/github/go-mod/go-version/mrmarble/zmk-layout-viewer)
 [![Vuln](https://github.com/MrMarble/zmk-layout-viewer/actions/workflows/vuln.yml/badge.svg)](https://github.com/MrMarble/zmk-layout-viewer/actions/workflows/vuln.yml)
-[![golangci-lint](https://github.com/MrMarble/zmk-layout-viewer/actions/workflows/golangci-lint.yml/badge.svg)](https://github.com/MrMarble/zmk-layout-viewer/actions/workflows/golangci-lint.yml)
+[![golangci-lint](https://github.com/MrMarble/zmk-layout-viewer/actions/workflows/golangci-lint.yml/badge.svg)](https://github.com/MrMarble/zmk-layout-viewer/actions/workflows/golangci-lint.yml)
+
+A **work in progress** Cli tool to generate preview images from a zmk .keymap file.
+
+## Usage
+
+```shell
+Usage: zmk-layout-viewer <keyboard>
+
+Arguments:
+  <keyboard>    Keyboard name to fetch layout.
+
+Flags:
+  -h, --help           Show context-sensitive help.
+  -f, --file=STRING    ZMK .keymap file
+  -t, --transparent    Use a transparent background.
+  -o, --output="."     Output directory.
+      --debug          Enable debug logging.
+```
+
+Keyboard name should be the same as in https://config.qmk.fm.
+
+```shell
+zmk-layout-viewer cradio
+```
+Will output this image:
+
+![](assets/layout.png)
+
+You can pass a .keymap file ([this one for reference](https://github.com/MrMarble/zmk-config/blob/master/config/cradio.keymap)) to generate the layout with bindings
+
+```shell
+zmk-layout-viewer -f ~/zmk-config/config/cradio.keymap cradio
+```
+will output an image for each layer:
+
+![](assets/default_layer.png)
+![](assets/left_layer.png)
+![](assets/right_layer.png)
+![](assets/tri_layer.png)