added a note about special characters in filenames

README.MD

@@ -58,6 +58,7 @@ See https://addons.mozilla.org/en-US/firefox/addon/single-file/statistics/?last=
   - The last saved path cannot be remembered by default. To circumvent this limitation, disable the option "Misc > Save pages in background".
 - Chrome/Opera:
   - You must enable the option "Allow access to file URLs" in the extension page to display the infobar when viewing a saved page, or to save a page stored on the filesystem.  
+  - The following characters are replaced with _ in filenames: ~, \, ?, %, *, :, |, ", <, >
 
 ## Troubleshooting unknown issues
 Please follow these steps if you find an unknown issue:

extension/ui/pages/help.html

@@ -470,6 +470,7 @@
 								https://addons.mozilla.org.</li>
 							<li>The last saved path cannot be remembered by default. To circumvent this limitation, disable the option "Save
 								pages in background".</li>
+							<li>The following characters are replaced with _ in filenames: ~, \, ?, %, *, :, |, ", &lt;, &gt;</li>
 						</ul>
 					</li>
 					<li>