[Predef-contribute] some updates on predef/README.md

[xim <xi...@ds...>]

Hi,

My name is Ming Xi. The predef package from github is very helpful for 
cross-platform programming.

I added the following to predef/README.md:

1. added DataModels.md to the index.

2. added a method to grab all pre-defined macros, and so user can handle 
new-platform easier.

Please see the attachment.

Yours sincerely,

Ming Xi