[erlang-patches] Add a proplist() type

[Henrik Nord]

Hi

Can you update the commit msg on your commit according to the rules on 
the wiki?

https://github.com/erlang/otp/wiki/Writing-good-commit-messages

Summarize clearly in one line what the commit is about

Describe the problem the commit solves or the use
case for a new feature. Justify why you chose
the particular solution.


  Add a proplist() type

  Recently I was adding specs to an API and found that there is
  no canonical proplist() type defined.


-- 
/Henrik Nord Erlang/OTP