online_editor: Some documentation in the README

Document the query arguments

tools/online_editor/README.md

@@ -1,9 +1,33 @@
+# Slint Online Editor
+
 This directory contains the frontend code for the online code editor
-which is hosted in https://slint-ui.com/editor
+which is hosted in https://slint-ui.com/editor (last stable) and
+https://slint-ui.com/snapshots/master/editor (nightly)
 
 To try it out locally type this in this directory:
 ```sh
 npm install
 npm start
 ```
 
+## Documentation
+
+The `index.html` page contains a code editor and every key press reload the preview.
+The `preview.html` page contains only the preview and the code must be given via query parameter.
+
+* `?load_url=`  query argument make it possible to load the .slint code directly from an URL.
+    If the slint code contains relative path for imports or images, they are loaded relative to
+    that slint file. That way it is possible to load  code from github (via raw.githubusercontent)
+    or gists.
+
+    Example: this loads the printerdemo.slint file from the github URL
+    - https://slint-ui.com/editor?load_url=https://raw.githubusercontent.com/slint-ui/slint/master/examples/printerdemo/ui/printerdemo.slint
+    - https://slint-ui.com/editor/preview.html?load_url=https://raw.githubusercontent.com/slint-ui/slint/master/examples/printerdemo/ui/printerdemo.slint
+
+* `?snippet=` query argument, followed by the URL-encoded slint code, will simply load this code
+   this is what is used tor the premalink feature
+
+   Example: a simple code with "Hello Slint"
+   - https://slint-ui.com/editor/?snippet=_+%3A%3D+Text+%7B+text%3A+%22Hello+Slint%22%3B+%7D
+   - https://slint-ui.com/editor/preview.html?snippet=_+%3A%3D+Text+%7B+text%3A+%22Hello+Slint%22%3B+%7D
+