Adding how to change passphrase in enterprise (circleci#464)

Author: kimh

jekyll/_docs/enterprise/faq.md

@@ -121,6 +121,17 @@ $ # To get private ip address
 $ ifconfig eth0 | grep "inet addr"
           inet addr:10.0.0.235  Bcast:10.0.0.255  Mask:255.255.255.0
 ```
+
+#### How can I change my passphrase?
+
+First, you need to change your passphrase from the system console (services box port 8800) settings page and restart your app.
+
+Then, you need to update `CIRCLE_SECRET_PASSPHRASE` in the init script that you are using to add builder boxes to your fleet. If you are
+using AWS ASG to manage your builder boxes, you can update the environment variable in the [ASG configuration]({{ site.baseurl }}/enterprise/faq/#can-i-use-aws-auto-scaling-groups-to-manage-my-builder-boxes).
+
+Now new builder boxes joining the fleet will use the new passphrase. Existing builder boxes with old passphrase also continue functioning without any real problems but we recommend rolling these box as soon as you can
+just in order to use the consistent passphrase in your fleet.
+
 #### I want to change my settings?  Can I do that?
 
 Yes!  Take a back up of `/etc/circle` -- edit the values in `/etc/circle/user`