Making pip require a virtualenv

pip-require-virtualenv

1575612896

Using python pip to install packages without using a virtualenv is generally
considered an antipattern.  Add this into your `~/.bashrc` to make pip
require an activated virtualenv before running.

```
# Do not pip install when not in a virtual environment
# https://docs.python-guide.org/dev/pip-virtualenv/#requiring-an-active-virtual-environment-for-pip
export PIP_REQUIRE_VIRTUALENV=true
```

Add note about pip require virtualenv
Add note about pip require virtualenv
