add overview

2019/2019-06-11_gitTraining/slides/overview.md

 ## Overview
 
-0. Subject 1
-1. Subject 2
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+0. The terminal
+1. The editor
+2. What is `git`? What is the use of `git`? <!--(5 min)//-->
+3. Installation of `git`
+4. GitHub and GitLab <!--(5min)//-->
+5. How do I configure `git`? <!--(5min)//-->
+6. Where and how to start <!--(5min)//-->
+7. What is a fork? <!--(5min)//-->
+8. What are branches?
+9. The 5 essential commands <!--(10 min)//-->
+    * `pull` / `status` / `add` / `commit` / `push`
+10. What are merge/pull requests? <!--(10 min)//-->
+11. How do I synchronize my fork?
+12. Best practices
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