Added GAE requirements doc in security

Added the Google App Engine requirements.
Hi Andrey,

Please check, I have updated the contribution file.

Thanks
Krishna Prasad


On Fri, Sep 5, 2014 at 9:12 AM, Andrey Petrov <notifications@github.com>
wrote:

> In CONTRIBUTORS.txt:
>
> > @@ -124,5 +124,8 @@ In chronological order:
> >  * Erik Tollerud <erik.tollerud@gmail.com>
> >    * Support for standard library io module.
> >
> > +* Krishna Prasad <kprasad.iitd@gmail.com>
> > +  * Google App Engine tests (https://github.com/krishnaiitd)
>
> I think you meant "documentation" instead of "tests"? Or was there another
> PR?
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/shazow/urllib3/pull/460/files#r17155910>.
>