added known issue (fix #251)

Former-commit-id: 509fdd45b8f91c32b61752558e1d778fc989ceae

extension/ui/pages/help.html

@@ -446,6 +446,8 @@
 							<li>For security reasons, you cannot save pages hosted on https://chrome.google.com,
 								https://addons.mozilla.org and some other Mozilla domains. When this happens, 🚫 is
 								displayed on top of the SingleFile icon.</li>
+							<li>For security reasons, SingleFile is sometimes unable to save the image representation of a 
+								canvas element.</li>
 							<li>The last saved path cannot be remembered by default. To circumvent this limitation,
 								disable the option "Misc. &gt; Save pages in background".</li>
 							<li>The following characters are replaced with _ in filenames: ~, \, ?, %, *, :, |, ", &lt;,
@@ -516,4 +518,4 @@
 	</div>
 </body>
 
-</html>
+</html>