Update readme.md to include links to data structure visualisation websites (#237)

Author: arathyrose

README.md

@@ -35,6 +35,10 @@ For testing your patch locally follow the steps given below,
 1. Install [pytest-cov](https://pypi.org/project/pytest-cov/). Skip this step if you are already having the package.
 2. Run, `python3 -m pytest --doctest-modules --cov=./ --cov-report=html`. Look for, `htmlcov/index.html` and open it in your browser, which will show the coverage report. Try to ensure that the coverage is not decreasing by more than 1% for your patch.
 
+For a good visualisation of the different data structures and algorithms, refer the following websites:
+- https://visualgo.net/
+- https://www.cs.usfca.edu/~galles/visualization/
+
 Why we use Python?
 ------------------