Point to spdx.org/licenses in license field docs

I expected the comment about the whitelist to say where you could find the whitelist. This mirrors the information given in the error message if you specify an invalid license identifier.
auto merge of #964 : carols10cents/cargo/patch-1, r=alexcrichton

I expected the comment about the whitelist to say where you could find the whitelist. This mirrors the information given in the error message if you specify an invalid license identifier.

I didn't make this a markdown link since it's within markdown within code within markdown, but I'm happy to edit this in any way you see fit.

Thank you!! :heart: