Split off ZRE-DISC protocol spec
Revert "Split off ZRE-DISC protocol spec"

This reverts commit a82b238ac7b3536e2e227c95d7eaa30567543e2f.
Clarifications to RFC4
Merge pull request #4 from hintjens/master

Clarifications to rfc.zeromq.org/spec:4/zpl