Update README.md to add step for installation of requirements.txt

[MarkTripod-Duo]

Description

Add pip installation step for requirements.txt to the README instructions

Motivation and Context

The explicit step to install the required python packages prior to execution of setup.py is needed to prevent potential installation errors that may occur when required packages call setuputils on their own.

How Has This Been Tested?

Manual testing of installation steps performed in a virtual environment.

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation

[ceckim]

Recommend adding instructions for setting up a virtual env:

Option 1: Use virtualenv

curl https://pyenv.run | bash

# Delete existing .pyenv if necessary
rm -rf ~/.pyenv

# Add stuff to your shell config
echo 'export PYENV_ROOT="$HOME/.pyenv"' >> ~/.zshrc
echo 'command -v pyenv >/dev/null || export PATH="$PYENV_ROOT/bin:$PATH"' >> ~/.zshrc
echo 'eval "$(pyenv init -)"' >> ~/.zshrc
source ~/.zshrc

# Install the desired python version
pyenv install 3.10.8

# Verify installation - this lists all installed versions & virtual envs for python
pyenv versions

# Create your virtual environment for python
# cd <your-project-folder>
# pyenv virtualenv <python version> <virtual env name>
cd ~/projects/duo_splunkapp
pyenv virtualenv 3.10.8 myProject310

# Install dependencies for your python project
pip install -r ../../bin/requirements.txt

# To activate a virtualenv
pyenv activate myProject310

# To de-activate the currently active virtualenv
pyenv deactivate

Option 2: Use venv

$ cd ~/src/duo_splunkapp
$ python3.7 -m venv venv
$ source venv/bin/activate
(venv) $ pip install -r bin/test-requirements.txt
(venv) $ make setup