Add menu entries and overview of rain sensor project

The main Project menu has a new entry for the rain sensors project
The main landing page has a brief overview of and link to the project

_includes/header.html

@@ -20,6 +20,7 @@
             <a class="dropdown-item" href="/projects/gate-alarm">Gate Alarm</a>
             <a class="dropdown-item" href="/projects/tongue-drum">Electro-Tongue Drum</a>
             <a class="dropdown-item" href="/projects/guitar-case-sensor">Guitar Case Humidity Sensor</a>
+            <a class="dropdown-item" href="/projects/rain-sensors/">Rain Sensors</a>
             <div class="dropdown-divider"></div>
             <a class="dropdown-item disabled" href="#">Kits</a>
             <a class="dropdown-item" href="/projects/kits/circular-led-chaser">Circular LED Chaser [Kit Bash]</a>

index.html

@@ -59,6 +59,12 @@ <h3>Guitar case humidity detector</h3>
         <p class="mb-2"><span class="badge badge-danger">Not&nbsp;started</span> <small><em>Click for more information.</em></small></p>
     </a>
 
+    <a href="/projects/rain-sensors/" class="list-group-item list-group-item-action flex-column align-items-start">
+        <h3>Rain sensors</h3>
+        <p>When we've got washing out it would be very helpful to be able to detect rainfall and to have an alarm sound so we can get the washing in. And while we're at it, we might as well measure the rainfall just to show how wet it really is in West Wales. This project aims to do both those things.</p>
+        <p class="mb-2"><span class="badge badge-warning">In&nbsp;progress</span> <small><em>Click for more information.</em></small></p>
+    </a>
+
     <a href="/projects/kits/circular-led-chaser/" class="list-group-item list-group-item-action flex-column align-items-start">
         <h3>LED Chaser Circuit kit bash</h3>
         <p>I bought a kit that I messed up building so decided to hack it into a circular LED chaser ornament that lives in a small perspex globe.</p>