Link to Druid-specific security page

Add a 'Security' link to the homepage pointing to the
Druid-specific security overview page, which contains
relevant information about the Druid security trust model,
and also contains the information on how to report security
issues.

src/pages/index.js

@@ -101,6 +101,13 @@ export default () => {
                     <a href="/docs/latest/tutorials/">Get started with Druid</a> in minutes. Load your own data and query it.
                   </p>
                 </div>
+                <div className="feature">
+                  <span className="fa fa-shield fa" />
+                  <h5> Security</h5>
+                  <p>
+                    Learn about <a href="/docs/latest/operations/security-overview.html">security features, configuration instructions, and best practices</a> to secure Druid.
+                  </p>
+                </div>
                 <div className="feature">
                   <span className="fa fa-question-circle fa" />
                   <h5> Get Help</h5>