added example of using previous release of core to test new xrefs
added example of using previous release of core to test new xrefs

james-monkeyshines/ensembl,at7/ensembl,danstaines/ensembl,james-monkeyshines/ensembl,Ensembl/ensembl,willmclaren/ensembl,at7/ensembl,Ensembl/ensembl,willmclaren/ensembl,james-monkeyshines/ensembl,Ensembl/ensembl,mjg17/ensembl,muffato/ensembl,at7/ensembl,muffato/ensembl,Ensembl/ensembl,mjg17/ensembl,mjg17/ensembl,danstaines/ensembl,mjg17/ensembl,muffato/ensembl,at7/ensembl,danstaines/ensembl,james-monkeyshines/ensembl,muffato/ensembl