Better document that `*_ipriming_sites.bed.tar.gz` contains the same data as `*.polyA.track`

[mapostolides]

hg38_ipriming_sites.bed.tar.gz can be downloaded but is named differently. the ".track" extension file is not available, which is a bit confusing. Thanks!

[mschilli87]

@mapostolides: What do you suggest us to do about that? Rename the file? Document this difference?
@Franzx7: Any reason for the name to change?

[mapostolides]

A small note in the documentation would be sufficient!

[mschilli87]

@mapostolides: I assigned this issue to @Franzx7 but feel free to open a PR yourself if you know how and have the bandwidth to do so.