David/translation/three first tutorial lessons

I just created the doc/en folder and added there the English translation of the first 3 french tutorial lessons. This tutorial has been a great help for me to understand how the Cesium app works and hope the translation could be helpful for other non french speaking developers. I'm not sure who to assign this merge request to, and who the reviewer should be. I hope I've done it the right way. If not, I will make the necessary corrections. I also added a link to the first translated lesson in the read me file.