Point at the demo from the top of the README

Somebody deciding whether to spend an evening on a mail client wants to
see it before they read about it. The link sits under the logo, above the
badges, because a demo answers the question the badges are only evidence
for.

The line under it says what the demo is -- invented mail, nothing kept --
so nobody arrives expecting to sign in with an address of their own.

README.md

@@ -2,6 +2,11 @@
   <img src="web/public/img/logo.png" alt="ihasmail" width="150">
 </p>
 
+<p align="center">
+  <strong><a href="https://demo.ihasmail.com">Try the demo</a></strong><br>
+  <sub>A working copy with an invented mailbox behind it — no sign-up, nothing real, nothing kept.</sub>
+</p>
+
 <p align="center">
   <a href="LICENSE"><img alt="Licence: AGPL-3.0-or-later" src="https://img.shields.io/badge/licence-AGPL--3.0--or--later-2dd4bf?style=flat-square"></a>
   <a href="https://stalw.art" target="_blank" rel="noreferrer"><img alt="Requires Stalwart 0.16 or newer; tested against 0.16.20" src="https://img.shields.io/badge/Stalwart-0.16.20-6366f1?style=flat-square"></a>