document usage

[SethTisue]

what version of Scala to use (and what resolver to add to get it), what dependency to add to your build to use the module, how to test your Scala version properly so 2.11/2.12/2.13 cross-building still works (#22), what import you need to get the needed conversions (import scala.collection.parallel.CollectionConverters._)...

this is a now task, not a someday task, since we are starting to have library maintainers actually use this (Scala.js is the first guinea pig)