added a bit about pairing options
added a bit about pairing options

Ensembl/ensembl-analysis,james-monkeyshines/ensembl-analysis,james-monkeyshines/ensembl-analysis,kiwiroy/ensembl-analysis,kiwiroy/ensembl-analysis,mn1/ensembl-analysis,Ensembl/ensembl-analysis,mn1/ensembl-analysis,james-monkeyshines/ensembl-analysis,Ensembl/ensembl-analysis,mn1/ensembl-analysis