Add link and reference to ProGit

README.md

@@ -9,6 +9,8 @@
 
 Examples and more information about bash-functions, bash-git_prompt and git-aliases below the Getting Started guide.
 
+More information about branching, forking, rebasing, merging and all that good stuff can be found in this free online 'book': https://git-scm.com/book/en/v2
+
 ### Getting Started with GitLab
 
 1. Generate an SSH keypair for GitLab (optional)