Adds docs for sponsor list

src/_content/styleguide/sponsor-list.html

@@ -2,6 +2,9 @@
 title: Sponsor list
 
 order: 100
+
+description: |
+  See `sponsor-listing.html` for the code that generates this list of sponsors. Most importantly, sponsor sizing is based on logo orientation. When adding logos, be sure to remove excess whitespace. Logos should be in SVG (preferred), WebP, or PNG format. If possible, SVG logo files should be optimized and minified.
 ---
 
 {% include "sponsor-listing.html" %}