added git-game link

added link to Daniel and Henry's git-game and fixed the following suggestions:

* added statement to explicitly state where to look for the next clue: 
  * **You should always check the README.md file for your next clue!**
* updated all README's to suggested writing style (it is a lot nicer on the eyes, thanks)
* fixed `who's` grammar mistake to `whose`
* removed unrelated text in `.gitignore` file
* corrected `abstract data structure` to `abstract data type`
* fixed  `the executable "outputclue"` to `the bash script outputclue.sh`  (each occurrence)
* explicitly stated there is a bug when merging and it needs to be fixed
* remember file is now 50k lines, 100k words, which prevents people from accidentally finding the solution and allows easy deciphering with diff.
* placed all information in the keyboard branch into the Henry branch (keyboard wasn't really needed)
* explicitly state to update master (also removed merge conflict). and is updated from forked repository

suggestions from: #646