Add top-level README product intro and free tempoll.app link (#27)

Author: felixgollnhuber

README.md

@@ -12,6 +12,14 @@
   Create the board. Share the link. Find the overlap.
 </p>
 
+<p align="center">
+  tempoll is a modern When2Meet-style scheduler: organizers create an event, participants join with just a name, and everyone edits availability on a live heatmap.
+</p>
+
+<p align="center">
+  Use tempoll for free at <a href="https://tempoll.app">tempoll.app</a>.
+</p>
+
 <p align="center">
   <a href="#quick-start"><strong>Quick start</strong></a>
   ·