Fix README.md

[MasashiYokota]

Thank you for this awesome project.

I've found that a part of README about Usage is a little out-of-date.
I've fixed the README based on this post of Elixir Forum.

If you know other better ways about this or have any questions, please give me some comments.
Thank you.

[whitfin]

Hi @MasashiYokota!

I'm fine with this, but I would like to keep the old example, since it's still valid. Maybe you can change it to something like this:

The typical use of Cachex is to set up using a Supervisor, so that it can be handled automatically:

Supervisor.start_link(
  [
    %{
      id: :my_cache_id,
      start: {Cachex, :start_link, [:my_cache, []]}
    }
  ]
)

If you are using Elixir versions prior to Elixir v1.5, you are able use the older syntax:

Supervisor.start_link(
  [ worker(Cachex, [:my_cache, []]) ]
)

[MasashiYokota]

@whitfin
Thank you for your comment.
I've modified README. Please confirm it.

[MasashiYokota]

@whitfin
I understand you are very busy, but I'm glad if you confirm my PR.

[krainboltgreene]

@whitfin How can I help get this published?