Adds note about getting https picture URLs (updates README.md)

Great module, thanks!  Just thought I'd pass along this comment in case it helps anyone else.  And, selfishly, so I remember how to do this next time I use this passport strategy :)

Source: https://developer.linkedin.com/forum/ssl-profile-picture-url-https

> Hi everyone,
> 
> We just pushed a fix for this today. Now you can indicate whether you want to fetch the secure or non-secure URL of the profile picture. By setting "secure=true" you're indicating that you want the secure version of the profile URL.
>
> Secure URL fetch:
>
> GET http://api.linkedin.com/v1/people/~:(picture-url;secure=true)
>
> Non-secure URL fetch:
>
> GET http://api.linkedin.com/v1/people/~:(picture-url)

Getting the non secure URL is the same call as before. We've also updated the documentation to reflect this change.

Thanks again!
Kamyar