docs: note about source maps in singleton mode (#414)

README.md

@@ -186,6 +186,8 @@ The loader inject styles like:
 
 Injects styles in one `<style></style>`.
 
+> ⚠ Source maps do not work.
+
 ```js
 import './styles.css';
 ```
@@ -271,6 +273,8 @@ The loader inject styles like:
 
 Injects styles in one `<style></style>` on demand (documentation above).
 
+> ⚠ Source maps do not work.
+
 ```js
 import styles from './styles.css';