add link to CPPZMQ tour to README

Problem: a comprehensive documentation of cppzmq is missing from this repo.

Solution: add a link to the very extensive tour of cppzmq by @brettviren.

Author: egpbos

README.md

@@ -92,6 +92,8 @@ See the `examples` directory for more examples. When the project is compiled wit
 API Overview
 ============
 
+For an extensive overview of the `zmq.hpp` API in use, see this [Tour of CPPZMQ by @brettviren](https://brettviren.github.io/cppzmq-tour/index.html).
+
 Bindings for libzmq in `zmq.hpp`:
 
 Types: