update contributing.md

[khareyash05]

Hey! I would like to add the following in contributing.md
Add the following -

1.Difference between GIT and GITHUB
2.How to clone,fork repository
3.How to create a branch and then use git push to push to repo
4.Create a PR
5.Squash commits in a single issue into one
6, Updating the forked and local repo as the updations are made in the upstream
I would like to work on this as a part of GSSOC'21