Add links to additional learning material

[ostueker]

In file reference.md add a section for additional learning material.

e.g.:

• https://swcarpentry.github.io/git-novice/
• https://pcottle.github.io/learnGitBranching/
• eBook "Pro Git": git-scm/book/en/v2
• Training pages from GitHub, GitLab, Bitbucket,

[ostueker]

good material on branching:

• Interactive Tutorial: https://pcottle.github.io/learnGitBranching/
• Sandbox-mode: can be used to demo/practice: https://pcottle.github.io/learnGitBranching/?NODEMO
• eBook "Pro Git": https://git-scm.com/book/en/v2/Git-Branching-Branches-in-a-Nutshell