Describe content constraints in showcase.yaml

Ideally the code can be more flexible, but right now things are looking
good with the current content. Describe what sort of content could make
things look bad.

static-site/src/components/splash/showcase.yaml

@@ -1,5 +1,9 @@
-# When adding or modifying descriptions, ensure it takes up at most two lines
-# when rendered on the splash page.
+# Notes:
+# - When adding or modifying names, ensure it takes up at most one line when
+#   rendered on the splash page.
+# - When adding or modifying descriptions, ensure it takes up at most two lines
+#   when rendered on the splash page.
+# - Ensure URLs start with `/`.
 cards:
 - name: SARS-CoV-2
   description: Ongoing evolution and spread of SARS-CoV-2