[css-color-5] Add algorithm for color-mix()

Author: LeaVerou

css-color-5/Overview.bs

@@ -126,9 +126,19 @@ Mixing colors: the ''color-mix()'' function {#color-mix}
 
 If a percentage is omitted, it defaults to 50% (an equal mix of the two colors).
 
+The result is produced via the following algorithm:
+
+1. If a percentage is omitted, it defaults to 50% (an equal mix of the two colors).
+2. Both colors are converted to the specified [=colorspace=].
 If the specified colorspace has a smaller gamut
 than the one in which the color to be adjusted is specified,
 gamut mapping will occur.
+3. Colors are then <a href="../css-color-4/#interpolation">interpolated in the specified colorspace</a>.
+The result of mixing is the color at the specified percentage along the progression.
+
+Note: As a corrollary, 0% just returns the first color converted to the specified colorspace,
+100% returns the second color converted to the specified colorspace,
+and 50% produces an equal mix of both.
 
 <div class="example">
 	This example produces a mixture of 40% <span class="swatch" style="--color: peru"></span> peru