Update README.md

Author: dwhitena

README.md

@@ -39,6 +39,7 @@
 - [Go 1.9+](https://golang.org/doc/install) - including GOPATH/bin added to your PATH (i.e., you can run Go binaries that you `go install`).
 - [Jupyter Notebook](http://jupyter.readthedocs.io/en/latest/install.html) or [nteract](https://nteract.io/desktop)
 - [ZeroMQ 4.X.X](http://zeromq.org/intro:get-the-software) - for convenience, pre-built Windows binaries (v4.2.1) are included in the zmq-win directory.
+- [pkg-config](https://en.wikipedia.org/wiki/Pkg-config)
 
 ### Linux