Update venv instructions in README

`venv` does not follow standard naming conventions for python virtual environments. Under __Quickstart__ the instructions should be updated to change `python3 -mvenv venv` to be `python3 -m venv .venv`. 
```
$ git clone https://github.com/ctsit/nacculator.git nacculator
$ cd nacculator
$ python3 -mvenv venv
$ source venv/bin/activate
$ pip install -e .
```