Specify sass-loader major version in example (#1539)

v11 is not compatible with webpack v4

Author: zgoda

README.md

@@ -330,7 +330,7 @@ The default templates comes with a `.css` file for each component. You can start
 
 #### [SASS]
 
-- `npm install --save-dev node-sass sass-loader` (inside your preact application folder)
+- `npm install --save-dev node-sass sass-loader@10` (inside your preact application folder)
 - start replacing `.css` files with `.scss` files
 
 #### [LESS]