Add Sift Security logo with link to blog post.

Signed-off-by: Misha Brukman <mbrukman@google.com>

Author: mbrukman

images/logos/siftsecurity.png

@@ -84,4 +84,5 @@ The following users have deployed JanusGraph in production.
 <a href="https://www.360.cn" class="logo"><img src="images/logos/qihoo_360.png" alt="Qihoo 360" class="logo" style="width: 200px"></a>
 <a href="https://www.redhat.com" class="logo"><img src="images/logos/redhat.png" alt="Red Hat" class="logo" style="width: 200px"></a>
 <a href="https://seeq.com" class="logo"><img src="images/logos/seeq.png" alt="Seeq" class="logo"></a>
+<a href="http://blog.siftsecurity.com/2017/09/using-security-event-graph-to-drive.html" class="logo"><img src="images/logos/siftsecurity.png" alt="Sift Security" class="logo"></a>
 <a href="http://denmarkblog.timesinternet.in/blogs/graph/times-internet-is-using-janusgraph-as-main-database-in-cms-for-all-newsrooms/articleshow/63709837.cms" class="logo"><img src="images/logos/timesinternet.png" alt="Times Internet" class="logo" style="width: 250px"></a>