docs: clarify handling of missing SVG files

Signed-off-by: Bjorn Lammers <[email protected]>

CONTRIBUTING.md

@@ -23,8 +23,8 @@ Thank you for your interest in contributing to the icon repository! To ensure sm
 
 ### Format
 
-- **SVG Format Required**: All icons must be submitted in SVG format.
-- **Automatic PNG and WEBP Generation**: PNG and WEBP versions are generated automatically from the SVG files using the following settings:
+- **SVG Format Required**: All icons should be submitted in SVG format. If an SVG version is unavailable, a PNG version will suffice, and a WEBP version will be accordingly.
+- **Automatic PNG and WEBP Generation**: PNG and WEBP versions are generated automatically from the SVG (or PNG) files using the following settings:
   - **Dimensions**:
     - Height: 512 pixels
     - Width: Auto (maintaining aspect ratio)