I actually tried leaving a note for myself in my house with a URL on it. Reading it later did not provide a link, just the url, which I could copy and paste into my browser.
Well, crud. I was hoping to be able to put a link to an image from the Glitch website in a Note at Ajaya Bliss showing where the rocks spawn. I guess people will have to copy it to their browser window. What makes me wonder why it's not possible is that you can post links in the chat window, but not a Note?
I've been doing this for some time - you can post a note with a url in it, however it won't be a clickable link. Simply ask the reader to copy paste it. I'd say try to keep it relative to glitch or linked within glitch, so irrelevant links don't ruin immersion for players.