Update README.md for bubble_sort

This is a:

• Update to an existing README.md

Details:

1. The current README.md doesn't have a brief description of the algorithm.
2. There is no example to explain how the algorithm goes on to sort the list/array.
3. The complexity of algorithm is not mentioned in the README.md.
4. There are no links if to refer if someone wants to read further about the algorithm.