Update installation instructions to include pip install awsebcli

**1. Please specify the following:**
- OS: [e.g. OS X, Ubuntu, Windows, etc.]: any
- Shell (Bash, PowerShell, Zsh, etc.): sny

- [x] **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/](https://pypi.org/project/awsebcli/)

Open the repository README and locate the installation instructions. Add the requested virtualenv guidance and `pip install awsebcli`, retaining the linked PyPI reference; done means the README clearly presents this installation path at the top and the link renders correctly.

Written by the indexing model from the issue text.