Update prepare_tx_gff.py

Coolio regarding [caddyhack #282](https://github.com/chapmanb/bcbio-nextgen/pull/282) - Here I just added some notes after noting `ensembl_realse = "73"`. Might be intentional or just a dust bunny...Overall http://beta.rest.ensembl.org/ looks like it could be super helpful for users wanting to add x.tropicalis takifugu rubripes and resolve these aliases into the ensembl "display name". Anyway, not sure if I should be making notes in your source without adding any actual code- but I'm learning some python along the way!