Fixed issue with saved directory being deleted & created README.md

Previously, if the user deleted the directory that was saved by BitmapEncoder, it would throw a NullPointer or IO Exception because there was no check if the file still existed when assigning the path to the FileChooser. This corrects that as it now checks if the directory exists and if not, sets the path for the FileChooser to the user's home directory. I also switched the information in 'How to run this.txt' into the README.md for Github.