Docs: Add note about Windows carriage returns.

Closes #1808.

README.md

@@ -131,6 +131,10 @@ When used locally, this feature requires that reveal.js [runs from a local web s
          data-separator-vertical="^\n\n"
          data-separator-notes="^Note:"
          data-charset="iso-8859-15">
+    <!--
+        Note that Windows uses `\r\n` instead of `\n` as its linefeed character.
+        For a regex that supports all operating systems, use `\r?\n` instead of `\n`.
+    -->
 </section>
 ```