Create cms-external-resources-collection.xml

Addressing https://github.com/cernopendata/opendata.cern.ch/issues/253
Missing screenshot integration and linking/addition of some input data
We cannot download images from `open-data-demo.cern.ch` because we are only creating it :) Please put original image location in an `FFT` tag, like we do for events:

```xml
    <datafield tag="FFT" ind1=" " ind2=" ">
      <subfield code="a">https://cms-docdb.cern.ch/cgi-bin/PublicDocDB/RetrieveFile?docid=12376&amp;version=1&amp;filename=BTau.ig</subfield>
      <subfield code="n">BTau</subfield>
    </datafield>
```