Add licenses plugin (#118)

source/plugins/licenses/README.md

@@ -0,0 +1,37 @@
+### 📜 Licenses
+
+    ⚠️ This is NOT legal advice, use at your own risk
+    💣 Do NOT enable this plugin on public web instances (plugin allows raw commands injection)
+
+The *licenses* plugin lets you display license informations like permissions, limitations and conditions along with additional metrics about dependencies.
+
+<table>
+  <td align="center">
+    <img src="https://github.com/lowlighter/lowlighter/blob/master/metrics.plugin.licenses.svg">
+    <details open><summary>With licenses ratio</summary>
+      <img src="https://github.com/lowlighter/lowlighter/blob/master/metrics.plugin.licenses.ratio.svg">
+    </details>
+    <img width="900" height="1" alt="">
+  </td>
+</table>
+
+Project must be setup with dependencies using `plugin_licenses_setup` option (for example, `npm ci` for a NodeJS project).
+
+Dependencies will be analyzed with [github/licensed](https://github.com/github/licensed) and compared against GitHub known licenses.
+
+#### ℹ️ Examples workflows
+
+[➡️ Available options for this plugin](metadata.yml)
+
+```yaml
+- uses: lowlighter/metrics@latest
+  with:
+    # ... other options
+    template: repository
+    user: repository-owner
+    query: '{"repo":"repository-name"}'
+    plugin_licenses: yes
+    plugin_licenses_setup: npm ci       # Command to setup target repository
+    plugin_licenses_ratio: yes          # Display used licenses ratio
+    plugin_licenses_legal: yes          # Display permissions, limitations and conditions
+```