Improve instructions, perhaps with diagrams

[yallop]

@kayceesrk noticed that the instructions are difficult to follow in places, and could be improved with examples or diagrams. For example, the instructions for shift_left_helper currently list various cases like this:

The first two squares are occupied by equal tiles. Merge them together, add an entry to the empties list, and process the rest of the row.

which sounds rather like medieval mathematics and could be made clearer with diagrams, perhaps like this:

shift_left_helper   ..    ..   ↝ shift_left_helper  ..   ..

[yomimono]

In #40 I've renamed empties to new_row, which might help a bit. I think the pictorial explanation is also a good idea.