ATLAS: new file `atlas-external-resources.xml`

* Creates new file `atlas-external-resources.xml`.
  (addresses #487) (PR #494)
docs: add new data files for installation

* Adds new data files to installation instructions.  (see PR #494)

* Cleans the wording slightly.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>