added requirements file for pip

[abhinand5]

Purpose of this PR
This PR fixes issue #532

Changes made
Alternative file requirements.txt was added.

[cuducos]

I would like to share my thoughts here. There is a reason we suggest Anaconda: it is simple and it help maintainers because of its simplicity.

This pip version suggested here would only work if the user already has a series of dependencies installed in their OS — this would make the system ready to compile under the hood requirements for those packages listed in the new file.

This adds complexity, expand the window for frustration in first-time users, and covering that would leave instructions longer and longer. Therefore, I advice current maintiners to be careful on this choice.

[abhinand5]

Fair enough.