[css-fonts-4] Add a helpful link to the list of default features

Author: litherum

css-fonts-4/Overview.bs

@@ -5193,6 +5193,7 @@ used to construct a combined list of font features that affect a given
 text run.
 
 1. Font features enabled by default are applied, including features required for a given script.
+        See [[#default-features]] for a description of these.
 
 2. <span id="apply-font-matching-variations">Font variations as enabled by the 'font-weight!!property',
 		'font-stretch!!property', and 'font-style!!property' properties are applied.</span>