Add video fot git internals to contrib file

CONTRIBUTING.md

@@ -84,4 +84,7 @@ write[*]Response: (input: Object) -> stream
 
 ### How git works
 If you want to contribute it may be usefull if you understand how git works under the hood.
-This is great article that shows the details: [A Hacker's Guide to Git](https://wildlyinaccurate.com/a-hackers-guide-to-git/).
+This is great article that shows the details:<br/>
+[A Hacker's Guide to Git](https://wildlyinaccurate.com/a-hackers-guide-to-git/).<br/>
+But as first the introduction you can watch this video:<br/>
+[![Link to Video: Inside the Hidden Git Folder - Computerphile](https://img.youtube.com/vi/bSA91XTzeuA/0.jpg)](http://www.youtube.com/watch?v=bSA91XTzeuA)