Nightscout Plugin (#108)

source/plugins/nightscout/README.md

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+### 💉 Nightscout
+
+The *Nightscout* plugin lets you display blood sugar values from a [Nightscout](http://nightscout.info) site.
+
+<table>
+  <td align="center">
+    <img src="https://github.com/legoandmars/legoandmars/blob/master/metrics.plugin.nightscout.svg">
+    <img width="900" height="1" alt="">
+  </td>
+</table>
+
+<details>
+<summary>💬 Setting up a nightscout site</summary>
+
+The [nightscout website](http://www.nightscout.info/) details how to self-host a nightscout site. Check out the instructions there.
+
+</details>
+
+#### ℹ️ Examples workflows
+
+[➡️ Available options for this plugin](metadata.yml)
+
+```yaml
+- uses: lowlighter/metrics@master
+  with:
+    # ... other options
+    plugin_nightscout: yes
+    plugin_nightscout_url: ${{ secrets.NIGHTSCOUT_URL }} # Use the github actions "NIGHTSCOUT_URL" secret as your nightscout site
+    plugin_nightscout_datapoints: 12                     # Use the latest 12 blood sugar datapoints to create a graph
+    plugin_nightscout_lowalert: 80                       # Blood sugars below 80 will be considered low
+    plugin_nightscout_highalert: 180                     # Blood sugars above 180 will be considered high
+    plugin_nightscout_urgentlowalert: 50                 # Blood sugars below 50 will be considered urgently low
+    plugin_nightscout_urgenthighalert: 250               # Blood sugars above 250 will be considered urgently high
+```