[css-snappoints] Copy in overview from Tab/Elika proposal, but making…

… snap position link to definition rather than inline for now (may merge the definition inline later).

css-snappoints/Overview.src.html

@@ -156,6 +156,30 @@ <h2 id="examples">Motivating Examples</h2>
         </figure>
     </div>
 
+<h2 id="overview">Overview</h2>
+
+    This module introduces control over <a>scroll snap positions</a>,
+    which are scroll positions that produce particular alignments
+    of content within a scrollable viewport.
+    Using the 'scroll-snap-type' propoperty on the relevant <a>scroll container</a>,
+    the author can request a particular bias
+    for the viewport to land on a valid <a>snap position</a>
+    during scrolling operations.
+
+    Valid <a>snap positions</a> can be specified
+    as a particular alignment ('scroll-snap-align')
+    of an element's <a>scroll snap area</a> ('scroll-snap-area', defaulting to its margin box)
+    within the <a>scroll container</a>’s <a>snap viewport</a>
+    (its viewport, as reduced by 'scroll-snap-padding').
+    This is conceptually equivalent to specifying the alignment of
+    an <a>alignment subject</a> within an <a>alignment container</a>.
+    The scroll position that satisfies the specified alignment
+    is a valid <a>snap position</a>.
+
+    <a>Snap positions</a> must only affect the nearest ancestor
+    (on the element's <a>containing block chain</a>)
+    <a>scroll container</a>.
+
 <h2 id="definitions">Definitions</h2>
 
     <dl>