Adding URL is too difficult, I had to click 20x to add simple URL of RSS feed. It should be like this: click on hamburger menu, choose "add feed", paste url, done.
Thanks for the feedback. In fact I hadn't worried about that because I thought that adding sources isn't something frequent even if it's a bit long at first and I thought that adding an "add source" button in the hamburger menu would be annoying since it wouldn't be used often.