Donlon version 1.1 from AIXM Wiki

Imported from:
https://extranet.eurocontrol.int/http://webprisme.cfmu.eurocontrol.int/aixmwiki_public/bin/viewrev/Main/%5BDonlon%5D%20Baseline%20data?rev=1.1
Donlon version 2.1 from AIXM Wiki

Imported from:
https://extranet.eurocontrol.int/http://webprisme.cfmu.eurocontrol.int/aixmwiki_public/bin/viewrev/Main/%5BDonlon%5D%20Baseline%20data?rev=2.1
Donlon version 3.1 from AIXM Wiki

Imported from:
https://extranet.eurocontrol.int/http://webprisme.cfmu.eurocontrol.int/aixmwiki_public/bin/viewrev/Main/%5BDonlon%5D%20Baseline%20data?rev=3.1
Donlon version 4.1 from AIXM Wiki

Imported from:
https://extranet.eurocontrol.int/http://webprisme.cfmu.eurocontrol.int/aixmwiki_public/bin/viewrev/Main/%5BDonlon%5D%20Baseline%20data?rev=4.1
Donlon version 5.1 from AIXM Wiki

Imported from:
https://extranet.eurocontrol.int/http://webprisme.cfmu.eurocontrol.int/aixmwiki_public/bin/viewrev/Main/%5BDonlon%5D%20Baseline%20data?rev=5.1
Donlon version 6.1 from AIXM Wiki

Imported from:
https://extranet.eurocontrol.int/http://webprisme.cfmu.eurocontrol.int/aixmwiki_public/bin/viewrev/Main/%5BDonlon%5D%20Baseline%20data?rev=6.1
Donlon version 7.1 from AIXM Wiki

Imported from:
https://extranet.eurocontrol.int/http://webprisme.cfmu.eurocontrol.int/aixmwiki_public/bin/viewrev/Main/%5BDonlon%5D%20Baseline%20data?rev=7.1
Donlon version 8.1 from AIXM Wiki

Imported from:
https://extranet.eurocontrol.int/http://webprisme.cfmu.eurocontrol.int/aixmwiki_public/bin/viewrev/Main/%5BDonlon%5D%20Baseline%20data?rev=8.1
Various improvements
Add LICENSE and README.md
Update LICENSE
Update README.md
Fix broken XLink references

These local references are broken and their UUIDs don't exist anywhere
in the Donlon dataset.  We are replacing these by abstract references
for now, to restore the referential integrity of the XML document at
least formally.