List requirements in readme

Would be great to know what the requirements are in terms of python and pip, particularly for people who aren't familiar with python. It took me over an hour to figure out the right combination of python and pip versions on Ubuntu 14.04 (I didn't even know pip was a thing when I started).

For anyone else in a similar boat, here are the requirements:

Python 2.7.6 (2.6 and 3.4 didn't work)
Pip 8.1.2 (ubuntu default 1.5.4 didn't work)
Use [virtualenv](http://docs.python-guide.org/en/latest/dev/virtualenvs/)

Here are the commands I ran to get it working:

``` bash
pip install --upgrade pip # to get 8.1.2 for Ubuntu 14.04 (YMMV)
git clone https://github.com/alex/letsencrypt-aws.git
cd letsencrypt-aws/
virtualenv -p /usr/bin/python2.7 venv
source venv/bin/activate
pip install -r requirements.txt
python letsencrypt-aws.py register me@mydomain.com
```