Add GitHub Skyline plugin (#147)

source/plugins/skyline/README.md

@@ -0,0 +1,27 @@
+### 🌇 GitHub Skyline 3D calendar
+
+    ⚠️ This plugin significantly increase file size, prefer using it as standalone.
+
+The *skyline* plugin lets you display your 3D commits calendar from [skyline.github.com](https://skyline.github.com/).
+
+<table>
+  <td align="center">
+    <img src="https://github.com/lowlighter/lowlighter/blob/master/metrics.plugin.skyline.svg">
+    <img width="900" height="1" alt="">
+  </td>
+</table>
+
+This uses puppeteer to generate collect image frames, and use CSS animations to create an animated rendering (GIF images are not animated in GitHub flavored markdown rendering which is why this design choice was made).
+
+#### ℹ️ Examples workflows
+
+[➡️ Available options for this plugin](metadata.yml)
+
+```yaml
+- uses: lowlighter/metrics@latest
+  with:
+    # ... other options
+    plugin_skyline: yes
+    plugin_skyline_year: 0    # Set to 0 to display current year
+    plugin_skyline_frames: 60 # Use 60 frames (half-loop)
+```