MINOR: Add Rolling Upgrade Notes to Security Docs
MINOR: Add Rolling Upgrade Notes to Security Docs

And added info about the krb5.conf file as we don't appear to mention that in the current docs

Author: Ben Stopford <benstopford@gmail.com>

Reviewers: Ismael Juma

Closes #625 from benstopford/security_docs

(cherry picked from commit 962aec1a78cac18608556ac99ab583cdc944ac3f)
Signed-off-by: Guozhang Wang <fae442ba222671b2ebdf7511c9ca77079c43031b@gmail.com>

racker/kafka,racker/kafka,racker/kafka,racker/kafka