docs: add an editor screenshot to the README

docs/screenshot.png sits right before the Features section, linked to
the app. Plain Markdown image syntax, so the VS Code preview renders
it too; the title bar's file name is painted out.

README.md

@@ -32,6 +32,9 @@ EdenText is a web-based, powerful word processor for everything from quick notes
 > on every commit — but expect occasional bugs, and keep backups of documents
 > you care about.
 
+[![EdenText editing a multi-column document](docs/screenshot.png)](https://edentext.app)
+Sample File from https://sample-files.com/
+
 ## Features
 
 - **Real page layout** — A4/Letter pages, margins, headers & footers, footnotes,