Add reference to description of NDC policies

[khaeru]

As identified by @OFR-IIASA in this discussion on #25:

The suggested publication, rogelj_indc_2017, is already part of the description; this describes the methodology.

The second publication is a reference we require for "which" policies we have implemented. This was supposed to be part of a paper published by a colleague at PBL. The paper is still not out, so I was wondering if there is another source we can quote. @volker-krey Possibly we can link to http://www.climatepolicydatabase.org/