Update documentation/README

[SimonLab]

"Our first/next step is to make the docs for https://github.com/dwyl/alog super beginner friendly." 👍
ref: dwyl/technology-stack#67 (comment)

see also https://github.com/dwyl/phoenix-ecto-append-only-log-example for an example on how to implement an append only logic with Phoenix.

Elixir v1.7 provide some new features for writing documentation:
from https://elixir-lang.org/blog/2018/07/25/elixir-v1-7-0-released/

Creating documentation with Hexdoc:
from https://hexdocs.pm/

• https://hexdocs.pm/hex/Mix.Tasks.Hex.Publish.html
• https://hexdocs.pm/elixir/master/writing-documentation.html