add a note about address linking

src/webui-embed.html

@@ -64,6 +64,10 @@
           <pre>xmrig --coin monero -o pool.townforge.net:<span id="pool_port"></span> -u 4xxxx/TF1M....</pre>
           You can also set just a Monero address or just a Townforge address, if you want to mine a single chain.
         </p>
+        <p>
+          Note that merge mining links your Townforge and Monero addresses. If this is a problem for you, use a
+          subaddress you only use for mining.
+        </p>
         <p>
           <a href="https://townforge.net/">Townforge</a>
           is a fork of Monero, with a builtin multiplayer city building game. All game actions are performed