Add some links to data structure visualisation websites

[czgdp1807]

Description of the problem

Some data structure visualisation links can be added to README under testing guide so that's easier for the developer to test their code efficiently.

Example of the problem

References/Other comments

[arathyrose]

Hi, this website: https://visualgo.net/en has pretty good visualisations in data structures and algorithms through animation. May I add it in the readme?

[czgdp1807]

Sure. Add https://www.cs.usfca.edu/~galles/visualization/ as well.