Add links to sub parts of documentation

A fuller contents list reveals the nature of each document, and makes a
long markdown file easy to navigate

README.md

@@ -106,7 +106,20 @@ In production:
 ## Documentation
 
 * [Mixin-sets](/docs/mixins.md)
+  * [Grid layout](/docs/mixins.md#grid-layout)
+  * [Conditionals](/docs/mixins.md#conditionals)
+  * [Colours](/docs/mixins.md#colours)
+  * [Typography](/docs/mixins.md#typography)
+  * [CSS3](/docs/mixins.md#css3)
+  * [Buttons](/docs/mixins.md#buttons)
+  * [Phase banner](/docs/mixins.md#-phase-banner)
+  * [Phase tags](/docs/mixins.md#-phase-tags)
 * [JavaScript](/docs/javascript.md)
+  * [Media player](/docs/javascript.md#media-player)
+  * [Multivariate test framework](/docs/javascript.md#multivariate-test-framework)
+  * [Primary links](/docs/javascript.md#primary-links)
+  * [Stick at top when scrolling](/docs/javascript.md#stick-at-top-when-scrolling)
+  * [Selection buttons](/docs/javascript.md#selection-buttons)
 
 ## Licence