Add description about Virtualenv in Contributing.md

[nj4710]

I think that the section of virtualenv in Contributing.md could use some more description about Virtual environments or a link to a page describing virtual environment. That will be very helpful for newcomers.
Installing virtualenv
[sudo] pip install virtualenv

Also, in some lines like

We use cssnext as a tool for compiling css

a full stop is missing.
So, what do you think?

[zoepage]

+1 on additional info to virtualenv. Do you have anything specific in mind, like a link that provides the information you'd like to see @nj4710 ?

Also +1 on adding a full stop.

[nj4710]

I would like to have a description like Virtual Environment is a tool to create isolated environments for different projects so as to prevent conflicts.

or a link to the docs https://virtualenv.pypa.io/en/stable/

[sunakshitejwani]

@nj4710
We can keep both the Description(with an example of where the conflict might occur and if it occurs how can it be solved) as well as the link.

[zoepage]

Also +1 on both :)
@nj4710 wanna send a PR? :)

[nj4710]

sent:)

[zoepage]

Merged ;)
High5!

[nj4710]

Thanks @zoepage ✋