Add screenshot plugin (#217)

source/plugins/screenshot/README.md

@@ -0,0 +1,26 @@
+### 📸 Website screenshot
+
+The *screenshot* plugin lets you take a screenshot from any website.
+It can be restricted with a [CSS selector](https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Selectors) or you can take a full page.
+
+<table>
+  <td align="center">
+    <img src="https://github.com/lowlighter/lowlighter/blob/master/metrics.plugin.screenshot.svg">
+    <img width="900" height="1" alt="">
+  </td>
+</table>
+
+#### ℹ️ Examples workflows
+
+[➡️ Available options for this plugin](metadata.yml)
+
+```yaml
+- uses: lowlighter/metrics@latest
+  with:
+    # ... other options
+    plugin_screenshot: yes
+    plugin_screenshot_title: XKCD of the day  # Section title
+    plugin_screenshot_url: https://xkcd.com   # Website url
+    plugin_screenshot_selector: "#comic img"  # CSS selector to take into screenshot
+    plugin_screenshot_background: no          # Remove page background
+```