Small edits to the gradel readme
Small edits to the gradel readme

Removed the "only Java 8" note, and pointed out the pre-built releases
over at igv.org.
amwenger/igv,amwenger/igv,igvteam/igv,igvteam/igv,igvteam/igv,igvteam/igv,amwenger/igv,amwenger/igv,amwenger/igv,igvteam/igv