Private pads description update

Hi,
I just made clear that HTTPS is needed to run Etherpad with Nextcloud and private pads through the API.

I realized that in DisplayController.php line 95, we have setcookie() with the HTTPS only parameter set. I running all locally and I wondering why Etherpad couldn't grant me access to pads, and It was because the session ID was not being sent.

Thank you for your work! 😄

README.md

@@ -67,7 +67,7 @@ example, you’ll have to set the cookie domain to `example.org` within
 the Ownpad settings.
 
 If you want to create *truly* private pads, you have to dedicate an
-Etherpad instance for Nextcloud. You will then configure Etherpad to
+Etherpad instance for Nextcloud **running both with HTTPS**. You will then configure Etherpad to
 restrict pad access via sessions and pad creation via the API.
 For this, you have to adjust your Etherpad configuration file
 (`settings.json`) as following: