Add note about https requirements

[PatrikStenmark]

I just spent time trying out SilverBullet by just running the Docker container without any reverse proxy or anything in front of it. I couldn't understand why the offline mode didn't work. Finally I found the output in the dev console explaining that the web worker didn't launch because I didn't use https. To avoid other people wasting time, I added a note about this in the docs for Synced mode and the Docker.md page.

[zefhemel]

I think this is listed elsewhere too, but clearly not clearly enough. Thanks!