add Introduction to Networking book (#4312)

by Dr. Charles Severance (University of Michigan)

free-programming-books.md

@@ -691,6 +691,7 @@
 * [GitLab Handbook](https://about.gitlab.com/handbook/)
 * [How to get started with open source](https://opensource.com/resources/ebook/how-get-started-open-source) (ePub & ODT)
 * [Innovation Happens Elsewhere](http://dreamsongs.com/IHE/IHE.html) - Ron Goldman, Richard P. Gabriel
+* [Introduction to Networking](https://do1.dr-chuck.net/net-intro/EN_us/net-intro.pdf) - Charles Severance (PDF)
 * [Open Advice: FOSS: What We Wish We Had Known When We Started](http://open-advice.org)
 * [Open source in Brazil](https://www.oreilly.com/ideas/open-source-in-brazil) - Andy Oram
 * [Producing Open Source Software](http://producingoss.com) - Karl Fogel