update README

Author: cderv

install-quarto/README.md

@@ -12,10 +12,11 @@ This action will:
 Inputs available
 
 * `version` - _optional_. If provided, the specific quarto version will be installed. Ex: `version: 0.3.71`
+  **Only available on `main` branch**
 
   ```yaml
     steps:
-      - uses: quarto-dev/quarto-actions/install-quarto@master
+      - uses: quarto-dev/quarto-actions/install-quarto@main
         with:
           version: 0.3.71
   ```