Make home page "Lorem Ipsum" text easier to find and edit

I have modified index.html and hero_unit.html so that they pull the "Lorem Ipsum" text from files in the _includes/custom folder. This way the users can modify all of their home page text in one place. 

The new files have clear names:
/welcome.html
/grid_item_left.html
/grid_item_middle.html
/grid_item_right.html

I have also added instructions on how to modify the front page text on README.md 

Thank you for creating this awesome theme!