Add documentation for "compressedIndexURL" #3
Description
As discussed in the main repo here, there is no documentation explaining the "compressedIndexURL" field for a track.
When a compressed FASTA ".fa.gz" is provided with "fastaURL", the FASTA index ".fa.gz.fai" should be provided with "indexURL" and the GZIP index ".fa.gz.gzi" should be provided with "compressedIndexURL". Otherwise, there are issues in viewing the BAM.