Document double-colon notation to autolink to Erlang types, callbacks, etc.

[eksperimental]

The notation should be explained under the Elixir sub-section in the README file.

I will take care of it.

Related issue: #2134

[josevalim]

To be clear, the notation is t:Module.type/arity, it just happens that module in Erlang have a leading colon too.

[eksperimental]

I will explain that rationale in the PR