[install] Document install process for alpha

[slack]

We need a common location for alpha instructions. Ideally, this would/could use helm, but should include all the info needed to stand up the deis alpha.

Relevant alpha components:

• postgres Alpha Requirements postgres#5
• etcd Alpha Requirements etcd#7
• Registry Alpha Requirements registry#1
• Basic Kubernetes Minio component minio#1
• deis/builder

[bacongobbler]

With deis/charts#30 this should be as easy as "stand up your kubernetes cluster, configure kubectl, install helm, run helm update then run helm install deis".