[css-color-5] add source

Author: svgeesus

css-color-5/Overview.bs

@@ -52,6 +52,8 @@ Introduction {#intro}
 		</figcaption>
 	</figure>
 
+	<!-- from http://tristen.ca/hcl-picker/#/clh/8/267/0023A5/F8E0D6 -->
+
 	Currently Sass, calc() on HSL values, or PostCSS is used to do this.
 	However, preprocessors are unable to work on dynamically adjusted colors;
 	all current solutions are restricted to the sRGB gamut