Add information for Devs in README.md

[czgdp1807]

Description of the problem

Currently, the README.md has very little information for developers/contributors.
IMO, the following should be added after https://github.com/codezonediitj/pydatastructs#installation

Testing
----------
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 less than 1% for your patch. 

Example of the problem

References/Other comments

[annu12340]

I would like to work on this issue

[czgdp1807]

Hi @annu12340
This has already been done by #97
You can try #112 or make some improvements to the README on your own by adding information which you think should be there.

[annu12340]

ok

[czgdp1807]

Fixed by #97