Add Sift Security logo with link to blog post.

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

Author: mbrukman

images/logos/siftsecurity.png

@@ -82,4 +82,5 @@ The following users have deployed JanusGraph in production.
 <a href="https://finc.com" class="logo"><img src="images/logos/finc.png" alt="FiNC" class="logo"></a>
 <a href="https://gdatasoftware.com" class="logo"><img src="images/logos/gdata.png" alt="G DATA" class="logo" style="width: 100px"></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>