Recommend pipx for a simple install process

**1. Please specify the following:**
- OS: OS X, linux, windows
- Shell: Most any

- [x ] **I have reviewed the troubleshooting tips described [here]() and they do not solve my problem**

**2. Description**
Add **pipx** as an install method in the README.

**Install instructions:**
1. Install [pipx](https://pypa.github.io/pipx/)
2. `pipx install awsebcli`

Using pipx is much simpler than manually managing python vitualenvs. Pipx is an established, stable, tool that drastically improves the installation process of many, python-based, cli apps.
Pipx all but eliminates the concerns mentioned here: [whats-the-advantage-of-this-mode-of-installation-instead-of-regular-pip-inside-a-virtualenv](https://github.com/aws/aws-elastic-beanstalk-cli-setup#51-for-the-experienced-python-developer-whats-the-advantage-of-this-mode-of-installation-instead-of-regular-pip-inside-a-virtualenv)

Update the README with pipx as an installation method, using the issue's pipx installation link and `pipx install awsebcli` command. Read the existing installation section first and confirm the completed documentation clearly explains how to install the EB CLI with pipx.

Written by the indexing model from the issue text.