docs: Add `pip` instructions to `README.rst`

[virajsazzala]

There is a gitscribe package released on pypi, and because of that, we can use

pip install gitscribe

to install and use the package.

It would be good to add this information to the README.rst file.

[AniruddhaGawali]

i can help in that can you assign it to me

[virajsazzala]

Yes, thanks for looking into this @AniruddhaGawali, I'll assign it to you!