Known issue with CentOS 6

As reported in #25

README.md

 This will transparently rewrite incoming and outgoing envelope addresses.
 Run `service postsrsd start` and `postfix reload` as root, or reboot.
 
+Known Issues
+------------
+
+- The Postfix package in CentOS 6 lacks the required support for TCP
+  dictionaries. Please upgrade your distribution or build Postfix yourself.
+