rss: Clarify that RSS content is the abstract

I'm still debating whether to include the full text of the post within the
RSS feed, since some of them may be substantial (like the one I'll be
posting soon that I've been sitting on for a couple weeks because I'm too
busy/lazy to do final editing).

* src/rss.sh: Add "(Read full post)" link.

src/rss.sh

@@ -70,7 +70,10 @@ gen-item()
   <author><![CDATA[$author]]></author>
   <link>$www/$slug</link>
   <pubDate>$date</pubDate>
-  <description><![CDATA[$abstract]]></description>
+  <description><![CDATA[
+    $abstract
+    <p><a href="$www/$slug">(Read full post)</a></p>
+  ]]></description>
 </item>
 EOF
 }