Added link to zmq installation instructions

Otherwise error may occur during build time:

```
  CXX(target) Release/obj.target/binding/binding.o
../binding.cc:28:10: fatal error: 'zmq.h' file not found
#include <zmq.h>
         ^
1 error generated.
make: *** [Release/obj.target/binding/binding.o] Error 1
```