LHCb: new file `lhcb-external-resources.xml`

* Creates `lhcb-external-resources.xml`.  (PR #465)
docs: new file `lhcb-external-resources.xml`

* Updates installation instructions after addition of new file
  `lhcb-external-resources.xml`.  (see PR #465)

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