Add hover title to spec list endorsements; Improve spec list vertical spacing (#437)

Author: brianhawthorne

assets/css/custom.css

@@ -1,5 +1,6 @@
 table.spec__table {
   width: 100%;
+  margin-top: 1.2rem;
 }
 
 table.spec__table thead {

layouts/partials/specs/spec_list.html

@@ -1,7 +1,10 @@
 <table class="spec__table">
   <thead>
     <tr>
-      <td>Title</td><td>Endorsed By</td>
+      <td>Title</td>
+      <td title="The following projects support these recommendations">
+          Endorsed By
+      </td>
     </tr>
   </thead>
   {{- range $.Paginator.Pages.ByTitle }}