Add Book Professional Development GitLab Handbook (#3341)

* Add Book Professional Development GitLab Handbook

* moved to Open Source Ecosystem and fixed warning

free-programming-books.md

@@ -606,6 +606,7 @@
 * [Free Software, Free Society: Selected Essays of Richard M. Stallman](http://shop.fsf.org/product/free-software-free-society-2/)
 * [Getting Started with InnerSource](http://www.oreilly.com/programming/free/getting-started-with-innersource.csp) (email address *requested*, not required)
 * [Getting started with Open source development](http://public.dhe.ibm.com/software/dw/db2/express-c/wiki/Getting_started_with_open_source_development_p2.pdf) (PDF)
+* [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
 * [Open Advice: FOSS: What We Wish We Had Known When We Started](http://open-advice.org)