Readme - install dependencies in virtual environment

[cpu]

Suggested by @krzysztofr in StreisandEffect/streisand#8

It is considered good practice to install any external python libraries in the virtual environment (using virtualenv, docs here), so you don't contaminate your system and have control over the different versions of libraries for different projects.
Such environment could be in any situation easily destroyed and recreated.

So, instead of, for example:
sudo pip install boto

You should create and activate virtual environment:

sudo pip install virtualenv
virtualenv venv
source venv/bin/activate

and install external dependencies locally, in the context of this environment:
pip install boto

Moreover, you can create list of all dependencies:
pip freeze > requirements.txt

...and then install them with one command:
pip install -r requirements.txt

If you find my comment useful I can modify Readme file or, if it is needed, provide more detailed explanation.

PRs welcome.