Add information about Ensembl transcripts to docs

[korikuzma]

We really only test RefSeq accessions in Cool-Seq-Tool (see #4). However, people such as the CIViC team use Ensembl. There have been questions on why providing Ensembl transcripts fails. We should document these limiations. See #87 for more information.

In addition to this, maybe we validate that provided accessions are RefSeq.