Update README, made it much more simple and clear
Update README, made it much more simple and clear

* Reworded a bunch of sections
* Added "features" section with bullets for the various features
  of Slate
* Added screenshot of Slate

linchpinio/docs,Ibercheck/api-docs,ProcessOut/documentation,ProcessOut/documentation,linchpinio/docs,linchpinio/docs,bookiza/docs,ProcessOut/documentation,bookiza/docs,Ibercheck/api-docs,bookiza/docs,bookiza/docs,claylo/api-docs,claylo/api-docs,Ibercheck/api-docs,kasko/docs,claylo/api-docs