Installation, setup, and `$VENV` path are in conflict

[stevepiercy]

@ergo readme.txt conflicts with the tutorial's first two pages:

• https://docs.pylonsproject.org/projects/pyramid-blogr/en/1.10.x/requirements.html#set-an-environment-variable
• https://docs.pylonsproject.org/projects/pyramid-blogr/en/1.10.x/project_structure.html

And those two pages are in conflict with each other because we now use a cookiecutter to get started and the path set for $VENV is wrong, having been set in the first page of the tutorial before the project has been created via cookiecutter in the subsequent step.

It's up to you to choose whether and when to set and use $VENV, ensuring the process and usage is consistent throughout the docs.