Adds a title to elaborate on how we count recent downloads.

Author: tcr

app/styles/app.scss

@@ -350,3 +350,8 @@ h1 {
 a.arrow svg {
     background: #EEECDD;
 }
+
+abbr[title] {
+    text-decoration: none;
+    border-bottom: 1px dotted;
+}

app/templates/components/crate-row.hbs

@@ -24,7 +24,7 @@
     </div>
     <div class="recent-downloads">
         {{svg-jar "download"}}
-        <span class='num'>Recent: {{ format-num crate.recent_downloads }}</span>
+        <span class='num'><abbr title="Downloads in the last 90 days">Recent:</abbr> {{ format-num crate.recent_downloads }}</span>
     </div>
 </div>
 <div class="quick-links">