doc: add many rules to readme

README.md

@@ -7,6 +7,10 @@ You can create many states in your `.env` and switch between them.
 Rules:
 
 - State name starts on a new line with `###` symbols (Ex. `### local`)
+- State name can contain multiple comma-separated sections (Ex.
+  `### local,staging`)
+- Each section may specify a namespace (Ex. `### debug:on,dev:on`). If a section
+  doesn't contain a namespace, it's a global namespace.
 - State ends if line is empty or contains a new state name.
 
 You can see the [full example].