Be clearer on dependency of zeromq

[viktorcarloskar]

Hi!

After running npm install zerorpc I got the Package libzmq was not found error constantly. Tried the linked "fix" continuously but without any results. As it turns out I did not have zeromq installed and therefor the installation failed.

The formulation of the installation process is quite unclear and since there is a "fix" to when the same error is thrown but it is thrown for different reasons (okay kind of the same reason, npm can't find libzmq, but the reason why it can't find it is different).

I feel the installation part of the package should be clearer with this.