Document installation procedure [JIRA: CLIENTS-692]

[lukebakken]

A fresh Ubuntu or CentOS machine will require many dependencies to be present to install the Riak Python Client. This needs to be documented.

[lukebakken]

Ubuntu Trusty docker file:

FROM ubuntu:trusty
ENV DEBIAN_FRONTEND noninteractive
RUN apt-get update
RUN apt-get install -y python-dev python-pip libffi-dev libssl-dev
RUN pip install cryptography riak
ENTRYPOINT /usr/bin/python