Update installation instructions to include pip install awsebcli

[theGASK]

1. Please specify the following:

• OS: [e.g. OS X, Ubuntu, Windows, etc.]: any

• Shell (Bash, PowerShell, Zsh, etc.): sny

• I have reviewed the troubleshooting tips described here and they do not solve my problem

2. Description
Please update the readme to include on the top the following instructions, which seem to resolve the many installation snafus with this repo:

create a virtualenv in a directory of choice

pip install awsebcli

anything else past this point is nice and all, but the pip package is just more reliable.
the package is also maintaned by amazon
https://pypi.org/project/awsebcli/