Replace code in Cookie Cutter template README.md

[FilippoBovo]

Replace this,

pip install -e {{cookiecutter.package_name}}
pip freeze | grep -v {{cookiecutter.package_name}} > {{cookiecutter.package_name}}/requirements.txt

with this,

pip install -r requirements.txt
pip install -e {{cookiecutter.package_name}}

Also add later on how to freeze requirements:

pip freeze | grep -v {{cookiecutter.package_name}} > requirements.txt