feat(docs): add portable mode feature to features page

Author: j4y

features.html

@@ -41,6 +41,9 @@
 <li><b>BMP to clipboard — Capture the magnified view</b><br />
 Double‑click inside the magnifier window to copy the current zoomed bitmap to your clipboard.</li>
 
+<li><b>Portable mode — Keep settings with the executable</b><br />
+Launch Color Cop with the <code>-portable</code> switch to store configuration files alongside the executable instead of in LocalAppData. Ideal for USB drives, VMs, and portable toolkits.</li>
+
 <li><b>Other minor features</b><br />
 • RGB Float mode with adjustable precision (Alt+[ / Alt+])<br />
 • Optional standard Windows crosshair cursor<br />