Added bubble sort implementation: Sorting/bubble_sort.py

Follows standard defined in Sorting/SORTING-README.md. min_value and max_value parameters not required as sort is comparative.
Added merge sort implementation: Sorting/merge_sort.py

Follows standard defined in Sorting/SORTING-README.md. min_value and max_value parameters not required as sort is comparative.
Updated sorting algorithm specification in SORTING-README.md

Algorithms now explicitly modify the list that's passed in, and return None. The four algorithms implemented so far (bubble_sort.py, insertion_sort.py, merge_sort.py, selection_sort.py) have been updated for this requirement.